For read operations, the following three consistency levels are possible:

• One: The lowest latency, but dirty reads are possible
• Quorum: The middle path
• All: The slowest, but data is always good

For write operations, the following three consistency levels are possible:

• One: The fastest
• Quorum: The middle path
• All: The slowest, but data is guaranteed to be written to all the replicas

Cassandra has more fine-grained consistency levels for both read and write, but the aforementioned three are mostly used. 

Consistency is a combination of the write and read, and you can have any combination of the two, for example:

• write one, read one
• write one, read quorum
• write quorum, read quorum (recommended)