How many chain links do I need?
The speed is the number of cogs on the rear cassette. This matters because higher numbers of gears (esp 9speed) mean that the chain must be narrower to fit between them.
Generally 6-7-8 speed chains are the same and 9 speed are thinner, more than 9 speed is a bit specialized.

The length isn't as important because you will usually have to shorten the chain to fit your bike. The standard chain is 114 pins (56 links) but you almost never need to know this ( I had to look it up!).