This crochet size and weight infographic details all the different yarn weights and the specifics of each. It breaks down the yarn type, weight, wraps per inch and ply. This infographic also includes a chart of different crochet hook sizes.
Crochet symbols & directions chart from Craftsy. Good article to read if you don't know how to crochet from charts. It's really a simpler way to work than from written instructions and it allows you to use all those gorgeous Japanese and Russian patterns!