CQL
The preferred interface to Cassandra 1.2 and 2.0 is CQL. DataStax has documentation for CQL and cqlsh, and the language grammar is here.
To use CQL you should also understand Cassandra data modeling.
If you plan to write a CQL client, you should also read about the communication protocol specification.
Thrift
You can read about the legacy Thrift RPC API here.