A saw chain is usually measured by pitch, gauge, and drive link (Part that sits in the groove of the guide bar/sprocket). The pitch is the space between each drive link, the gauge is the thickness of the drive link, and the number of drive links will usually tell you the length of the chain.
And I will note that chains are not standard. What that means is that an 18" chain for an Echo, will not fit a STIHL. I know Poulan has about 2-3 different kind of 20" chains and different brands utilize different chains also. You can have the same pitch and gauge, but if the number of drive links are different, then it will be either too long, or too short.
For smaller saws, it will usually have a 3/8" Picco pitch. (STIHL measurement), and a .43 or .50 gauge drive link.
Mediums saws usually have .325 pitch and .50/.63 gauge.
Bigger saws usually use 3/8" or .404 pitch. STIHL runs .63 gauge, unless Echo/Husqvarna/Poulan which are usually .50 gauge.
What this all means is simply put, you can't have a .325 pitch chain on a 3/8" sprocket, or will keep jumping off. Also the gauge needs to be correct, or else it will not even fit on the bar.
Most smaller to mid-size Poulans use 14"-18" bars, 3/8" P, .50 gauge. 14" would be 52 DL's, 16" would be 56 DL's, and 18" would be 62 DL's. You can any brand of chain, as long as those measurements match up.
How do you find this all out? Research. Invest time into learning wha you model needs. If you look on the side of the bar, usually closer to the bar mount, it will have some numbers engraved/stamped. That usually will point you in the right direction.
If not, you can remove your old chain and count the number of drive links, different companies have differnt links.
The reason the drive links look different is because different manufacturers design them differently. STIHL chain has a ramp to deliver oil into the chain, Oregon might have a ramp/groove, but I'm not sure.
This is all very important since safety is a concern. If you try and cobble together a saw from different parts from different manufacturers you will quickly see how bad of an idea it was. Much like Dr. Frankenstein you will understand why things are the way they are. In a best case scenerio the slop will quickly ruin both bar and chain.91 on the drive link would indicate 50 gauge but his is 90 chain(.043).
Understand you tools. Everytime you are using them it is a matter of life and death no matter how well prepared you are.
Minimize all risks as best you can or find somebody else that is more expeienced to opporate the chainsaw.
