The question of longboard vs skateboard is very highly argued. Both sports feature a board with 4 wheels, trucks, bearings, hardware, and griptape, but each one is completely unique.
Longboarding is built around speed, cruising, and playing around on the board using roads as opposed to features (stairs, rails, etc.). Skateboarding is similar in many ways except not commonly for speed. The main difference comes down to one being made for air and rail/grind based tricks, and the other for speed and different types of tricks and cruising.
It's important to note that both sports started in the same place and as more designs come out, the lines are being blurred in many ways. Though the rivalry can run deep in many, both sports can be very gnarly and dangerous when pushed to the limits.
SKATEBOARD VS LONGBOARD
Skateboarding is often called street skating. Skaters can be as creative with their lines and tricks as they want because the boards are lightweight and built with a double kick-tail. This double kick lets a skater do hundreds if not thousands of different tricks and combinations and makes it great for grinding and any trick that involves an ollie.
It's generally the more common of the two. Skating with a lot of speed can be far sketchier compared to longboard. The trucks are generally set a bit loosely and are narrower with smaller wheels. This makes for a less stable ride and causes the wheels to catch on small rocks in the road.
One awesome part about skateboards aside from how great they are for tricks, is how portable a skateboard is. This is why mini cruiser longboards are becoming more and more commonplace. Skateboards are also less expensive and diverse in design making it easy to choose and start skateboarding.
Most people start out on a skateboard due to notable athletes in the sport like Rob Dyrdek and Tony Hawk. It's probably easier to start on a cruising longboard than a skateboard since they are more forgiving with rocks and bumps on the road.
LONGBOARD VS SKATEBOARD
Longboarding is about speed, carving, and slide. The boards are (generally) longer with wider trucks and larger wheels. This makes them far more stable at speed and easier to balance when learning. These are more technically advanced and built in 1000's of different ways.
The top longboarders are pushing the limits and have set speed records that are pushing close to 100MPH. These guys wear full on leather speed suits and full face helmets in case they crash. Don't let this scare you off however, they really are the easier board to learn to skate with.
Longboarding is divided into many disciplines and has a ton of variety in board styles for each. Just some of the disciplines are freeride, dance, freestyle, downhill, cruise, carve, commute, push, etc.. Among each style exists even more board designs that are specific or aimed at filling the gap between several. Check out this article to learn more about the different longboards.
Recently, more and more longboards are being built in a surprising way, short! These are typically called mini cruisers and sometimes feature a single kick tail to make the board more adaptable for cruising on paths and popping up and down curbs. This hybrid is somewhat a best of both worlds for someone that isn't serious about skating park, or bombing big hills. They have bigger wheels to make them more forgiving than a skateboard.
WHICH ONE IS FOR ME?
Are you just starting out and wanting to use your board on streets and cruising around with friends? Then a longboard is the place to start. It's also what you want to use if you love speed or want to skate hills and neighborhoods.
If you're goal is to ollie, learn to skate park, or hit rails, than a skateboard is clearly the choice. It's best to start with the board that works best for what you ultimately WANT to do.
Choosing to skateboard vs longboard is a tough decision, but keep in mind that both require a similar skillset. This makes it easy to pick up both and be able to skate park or cruise hills. Don't limit yourself to thinking one is better than the other, each is a ton of fun and lets you take as much risk as you could ever want.