Where can I find descriptions of arXiv categories?
From the arxiv.org home page, click on detailed description for the discipline that you need (everything except physics has one). For instance, the one for computer science reads:

LG - Machine Learning [...]
Covers machine learning and computational (PAC) learning. Roughly includes material in ACM Subject Class I.2.6.
CY - Computers and Society [...]
Covers impact of computers on society, computer ethics, information technology and public policy, legal aspects of computing, computers and education. Roughly includes material in ACM Subject Classes K.0, K.2, K.3, K.4, K.5, and K.7.