Chain pitch is the size of the chain and is defined as the distance between any three consecutive rivets divided by two. Chain is made in several pitches - 1/4", .325", 3/8" Low Profile, 3/8", .404". Pitch is important because the drive sprocket and bar nose sprocket must be the same pitch as the chain.
Chain Gauge is the Drive Link's thickness where it fits into the guide bar groove, matching the guide bar gauge. The gauge of the chain and the gauge of the bar must match. The common gauges of saw chain are: .043", .050", .058", and .063".
Once you have identified the pitch and gauge the next thing you need is the number of links. You will need to count the number of drive teeth.