Most trucks fall into either the compact or full-size category, though "compact" is a relative term nowadays. The Detroit-label full-size trucks are also available in heavy-duty guise.
Though each manufacturer usually has its own snazzy name for them, there are three basic body styles for trucks: regular cab, extended cab and crew cab. Regular cabs have two doors and seat between two and three. Compact extended cabs add mostly extra storage space, while full-size versions can seat three additional passengers. Rear clamshell doors make it easier to get people and cargo in and out of the back of extended cabs, though a few have small, front-hinged doors. Crew cabs have four full-size doors and seat between five and six passengers comfortably.
Base-model compact trucks are some of the cheapest vehicles on the market, but loaded premium models can run into the low $30,000s. Full-size trucks range from the low $20,000s to the low $50,000s.
Compact trucks offer four-, five-, and six- and eight-cylinder engines for a variety of different buyers. Many full-size trucks offer six-cylinder engines on base models, but most trucks use V8 or even V10 engines that return marginal fuel economy numbers at best. Heavy-duty full-size trucks offer diesel engines that provide plenty of towing power and better mileage.
Most trucks offer basic safety equipment like front airbags and seatbelts for all passengers. Side curtain airbags and stability control are now commonplace on full-size models. Some compact models perform poorly in crash tests, and one should be mindful of rear jump seats that aren't designed to protect someone in a crash.
Almost all trucks offer both manual and automatic transmissions, as well as two- and four-wheel drive. Many also offer traction-aiding devices like limited-slip/locking differentials and electronic traction control.
Most trucks can be had with a three-person front bench or bucket seats. Extended or crew cabs offer the ability to either fold the entire rear seat up or at least one half or the other. Some newer full-size trucks also allow you to fold the front passenger seat flat to provide a handy tablelike surface.
Compact trucks can tow between 3,000 and 7,000 pounds, depending on how they're equipped. Regular full-size trucks can tow as much as 12,000 pounds with certain engines and suspension configurations, although most range between 8,000- and 10,000-pound capacities. Heavy-duty full-size trucks can haul as much as 30,000 pounds when equipped with a fifth-wheel connection. Be mindful of axle ratios, as they play a big role in towing capacity as well as fuel economy.
Compact trucks are relatively fuel-efficient and cheap to insure because of their low purchase price. As you add size and cylinders, expect a corresponding increase in fuel and insurance costs. Adding four-wheel drive to any truck will add to annual operating costs as well. As trucks add features, more modern engines and thus complexity, they aren't as accessible to the home mechanic as they once were.