Here’s a really good document describing the different series and the various paybacks you have to give to your investors for each round:

https://blog.ycombinator.com/how-to-raise-a-seed-round/

Here is a lighter document describing the purpose of each investment round with respect to what the company should be doing:

http://www.startupfreak.com/what-does-series-a-series-b-series-c-funding-mean-in-startups/