Great question! A lot of online skate shops will simply show you a picture of a board, with the width, and then some random other numbers. When you see something like this 143-k13, what does it mean?
This first number tells the shape of the skateboard deck. Skate One Manufacturing has numbered each shape (the "Shape" of a skateboard includes it's concave, width, length, wheelbase, and the length of the nose and tail). They have numbered every single combination that they sell. You can look at this chart on the Skate One website.
The "K13", or K - some other number, refers to the concave. But what does that mean? Well, the concave is how much the skateboard bows in in the middle of the deck. Skateboards aren't usually made totally flat - they are built with concave. This makes them a lot stronger, and gives them more pop.
The letter "K" means that it likely looks like a regular skateboard, and the higher the number, the deeper the concave. An "s" after the number (for example "K12s") is a shorter skateboard, made for smaller skaters. You can see an entire Chart of Skate One Concaves.
This should help you understand a bit better what you are looking at when you see these letters and numbers listed with skateboards you are thinking about buying.