Most trucks fall into either the compact or full-size/light-duty category, though "compact" is a relative term nowadays. The Detroit-label full-size trucks are also available in heavy-duty guise.
Though each manufacturer typically 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. Most extended cabs now have regular, front-hinged doors, but less convenient rear clamshell doors are still possible. 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. Light-duty trucks range from the low $20,000s to the low $50,000s.
Compact trucks offer four- and six-cylinder engines for a variety of different buyers. Most full-size trucks offer six-cylinder engines on base models, but most use V8 engines that return marginal fuel economy numbers at best. Diesel engines that provide plenty of towing power and better mileage have migrated from the exclusive realm of heavy-duty trucks down to compact and light-duty models.
Trucks now offer basic safety equipment like front and side airbags, side curtain airbags and stability control. However, like cars, they also come with extra equipment like rearview cameras, trailer sway control and advanced accident avoidance tech. Also make sure to consider crash scores from NHTSA and the IIHS.
The manual transmission has largely been eliminated from the truck world, leaving behind automatics that can have as many as eight gears. As always, two- and four-wheel drive are available. 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.
Compact trucks can tow between 3,000 and 8,000 pounds, depending on how they're equipped. Regular light-duty trucks can tow as much as 12,000 pounds with certain engines and suspension configurations. 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.