Riding pants and jerseys provide reasonably comfortable protection for your limbs and torso when operating an off highway vehicle. You will see people wearing less specific items like jeans and t-shirts but they are unnecessarily risking injuries that can come from just riding, let alone from accidents. I didn't have real riding gear for my first few outings and I quickly learned the limitations of jeans and casual shirts. Jeans and other work pants are not designed for you to straddle the seat of a motorcycle. You will quickly discover they get tight in very uncomfortable places and lack the flexibility you need for comfort and proper movement on the bike. Short sleeved shirts will expose your arms to sunburn and scratching by brushes or branches along the trail. Sweatshirts get too warm after only a few minutes of riding on even cool days.
Riding pants are sometimes called "leathers". That is because the riding pants originally worn by old time motorcycle riders were usually made of leather to provide warmth in cold weather and protection against abrasions when they parted company with their machines. These days they are usually made of sturdy nylon like Gore-tex, augmented by plastic or silicone protective patches that often mascarade as logos or design elements. Riding pants are usually equipped with soft hip pads that fasten to velcro tabs on each hip the the better ones include pockets for knee pads that also cover the shins.
Riding pants should fit comfortably and not bind when you are sitting, standing, or walking. Some have adjustable waists. Make sure they fit snugly around your waist. You don't want them lose enough to slip down!
A lot of riders simply wear jeans or sturdy work pants instead of riding pants. True, it may be cheaper to wear available pants but they won't provide the level of comfort or proper protection of real riding pants.
Riding "shirts"are called jerseys. They are usually made of a fairly light and breathable but sturdy material so they can provide some protection against dirt and sand kicked up at trail speeds. They need to fit comfortably and allow full freedom of movement of your arms. Some jerseys have built in soft elbow pads to give a little extra protection but strap-on, hard elbow pads will give even more protection. For cold weather riding, choose "Wind Chill"jerseys, designed to help keep you warmer in cooler weather.
You will definitely see guys riding wearing a variety of tops -- t-shirts and even tank tops in hot weather and sweat shirts when it is cooler. However, once again, these garments do not provide the comfort, flexibility of movement, nor the protection of a proper riding jersey.
Riding pants and jerseys were at one time frequently color coordinated to the rider's machine. While the practice is still fairly common and is a good way to show your support for your favorite brand of equipment, more colorful options have become popular, very likely fueled by the exotic color schemes adopted by Supercross riders over the years. It isn't unusual to find very macho riders even wearing a combination of pink and chartreuse.
Once again, you might get away with wearing jeans and a t-shirt or sweatshirt for a while, but real riding pants and jerseys will definitely be more comfortable and safer. And it looks cooler!