Most tents these days, except pup tents and military "shelter halves", have sewn in floors.
Having a sewn in floor helps maintain the shape of the footprint of the
tent and prevents drafts and critters from slithering inside when the
door is zipped closed. However, tent floors are generally not very
sturdy nor completely waterproof so you always benefit from using a good ground
cloth beneath your tent. I like to use one inside my tent too. A
ground cloth beneath your tent will help protect the floor from debris
and moisture. One inside the tent helps protect the floor from damage
from walking on it or setting heavy objects on it and gives you
and your sleeping bags extra protection against ground moisture. I've seen oversize outside ground
cloths carelessly installed such that moisture dripping of the tent was
caught on the top of the ground cloth and ran under the tent, largely
defeating one of its major purposes. If you use a ground cloth beneath your tent, make sure it doesn't extend beyond the tent walls. One common practice among tent campers is to dig a small trench around the outside of the tent to collect any water that runs off, but that isn't always practical. You won't want to trench around your tent when it is pitched on your lawn or the grass in a campground.
Many tents have floors made of the same light weight material as the walls and roof. That
might be good in a back packing tent where weight is a primary concern,
but for larger family tents used for car camping having a sturdier
floor will usually yield better protection and last longer, if you can find one with a sturdier floor. Some tents do have sturdier "bath tub"floors. Some I have seen appear to be better at retaining water that leaks through the tent than in keeping ground water out! For the most part you'll still need to add ground cloths to get the extra protection you need.
Most dome tents have built in floors. Some larger cabin tents might not. Regardless of what kind of floor, if any, your tent has, you will want to put down a ground cloth under your tent. If it has a sewn in floor the ground cloth will protect the fabric from being damaged by rocks, twigs, and other debris. If it doesn't have a floor at all, the ground cloth becomes the floor. I like to use two ground cloths -- one under the tent to protect the floor from debris and one inside the tent to protect the floor from walking on it. Having two ground cloths also helps prevent any moisture from coming through and soaking your sleeping bags or clothing and equipment left on the tent floor.
No matter what kind of floor, if any, your tent has, you'll want to prepare the ground before you set up your tent. Remove rocks, twigs, and other debris that might damage the tent or be uncomfortable to walk on or sleep on. A leaf rake with a collapsible handle is a good tool for clearing the ground. You might need a shovel to smooth out lumps in the dirt. Try to avoid damp places that will let moisture accumulate under your tent or seep inside. An ideal spot for a tent would be one that is a few inches higher than the surrounding ground so that rain or dew will drain away and not under the tent. Grassy areas make a soft pad beneath your tent, but leaving a tent set up on grass for more than a day or so will kill the grass underneath it. Sometimes you will notice the grass is wilted and starting to turn yellow after taking down a tent that has only been set up for a few hours. Grass itself contains moisture that can condense on the underside of the tent floor and might wick through.
For a really luxurious tent floor cover the floor with foam "anti-fatigue" foam tiles like are normally used in front of a workbench. They come in 2'x2' squares, are about 1/2" thick, and connect together like pieces of a jig saw puzzle. They
usually come in sets of 4 so yo may need multiple sets to cover the
entire tent floor. For example, you'll need 10 sets of 4 to cover the entire floor of an 8'x10' tent. That could cost around $75 but if you watch for sales at home centers or Harbor Freight you and sometimes get them for $5-10 a set, bringing the cost down to as little as $50. They will protect the floor, give an extra buffer
against cold, damp ground, and provide extra padding for sleeping
comfort. You could use them beneath your tent to protect the floor from debris on the ground but I prefer using them inside where they protect the floor from items in the tent poking through. Having them inside also keeps them cleaner, making it easier to pack them up and use them again. Here is what to look for in your favorite home center:
If you don't care for the industrial look of the standard
gray-black mats you can usually find brightly colored ones at home
centers. You might pay a little more for them but they will not look so industrial and might be more fun to use.
If your tent isn't an even multiple of 2' in any direction they can be easily cut to size. They are light weight but a little bit bulky. You won't want to take them back packing, but they can add level of comfort in base camp and when car camping. Here are some examples of the standard industrial grey/black color and brightly colored versions you can usually find at home centers. You'll pay a little more for the colored ones, but the mood boosting contribution of the colors may be well worth the additional cost, especially if your spouse has a penchant for color coordination or interior decorating.
As in the colored tiles above, sometimes the foam tiles have straight finished edges but many have interlocking nubs all around like the grey tiles in the picture above the colored tiles. Sometimes they come with edges that lock into the nubs to give a finished straight edge. If not, you can still use them as-is or cut the nubs off the outside edges.
Just about any tarp can be used as a ground cloth outside or
inside the tent for an extra layer of protection. The heavier the tarp,
the more protection. A sturdy canvas tarp will provide stronger
resistance to damage from rocks or sticks beneath the tent, but may be
heavier to carry and stiffer to lay out. Light weight blue "poly tarps"
are better than nothing, but the heavier silver tarps or green "farm"
tarps will be sturdier and last longer. Try to match the size as
closely as possible to the footprint of the tent. If it is too large it
can be folded under so it doesn't trap moisture running down the sides
of the tent. If it is too small you may have exposed edges where debris
can still damage your tent floor and expose you and your equipment to
the cold, camp ground. One that is just an inch or so smaller than the base of the tent should be just about right.
More on ground cloths. The sturdier the material, the better. Light weight "poly" tarps are inexpensive and are definitely better than nothing, but a heavier, canvas tarp will provide more protection. When putting a ground cloth beneath your tent make sure it doesn't extend beyond the edges of the tent. If it does, rain or dew dripping off the tent will collect on top of the ground cloth under your tent effectively negating one of the main purposes for using a ground cloth. As I mentioned before, I like to use a second ground cloth inside my tent to ensure my sleeping bags and other gear stays dry. It will also protect the sewn in floor against damage from walking on it and from items dropped on it. If you have one heavy tarp and one light one, use the heavy one beneath the tent and the lighter one inside. If you're camped where it won't damage the grass, dig a shallow trench around the outside of your tent to catch any moisture that runs off and channel it away so it doesn't seep under your tent.
Keeping the floor clean. You can minimize damage from walking on your tent floor by keep it clean. Tracking mud and other debris inside deposits stuff that can grind into the fabric and cut the fibers. Use a rug or an old burlap bag outside the door to clean your feet before entering. Sweep out your tent at least daily. When I'm using a dome tent, I usually pick it up and shake out all the dirt when I get up each morning unless it is staked down. If you have a cabin tent or your dome tent is securely staked down, sweep it out. There are small "camping" brooms made just for the purpose or you can cut down a regular broom or use a whisk broom. The shorter handle is usually necessary because of limited height inside smaller tents and it makes it easier to transport it. Whisk brooms or shop bench brushes can also be used but will require a lot more bending over, which can be hard on your back.
Floor it!