CQL
Cassandra offers a SQL-inspired interface called CQL. The primary difference from SQL is the emphasis on denormalization (via Collections) over Joins.
There is a quick introduction in GettingStarted and full documentation here.
To use CQL effectively you should also understand the [DataModel].
If you plan to write a CQL client, you should also read about the communication protocol specification. Clients do not need to parse CQL, so you shouldn't need to know the details of the antlr grammar
Thrift
You can read about the deprecated Thrift RPC API here.