The Oregon Chain Guide explains about chains.

What you need depends on how your saw is set up. Your pitch will probably either be 1/4, .325 or .375. I'd guess they would put either 1/4 or .325 on an electric saw.

The best way to find the pitch is hopefully your bar is stamped with numbers that will indicate what it wants. The bar or a lookup of the part number will tell you what pitch the bar can handle (assuming you have sprocket in the nose), what gauge the groove is and how many links you need. My guess is that the 55, 56 and 57 you saw at the store is the number of links in each of those chains.

Your current chain may have some markings on it that indicate what pitch and gauge the chain is.