A stream is a partitioned, immutable, append-only collection that represents a series of historical facts. For example, the rows of a stream could model a sequence of financial transactions, like "Alice sent $100 to Bob", followed by "Charlie sent $50 to Bob".
Once a row is inserted into a stream, it can never change. New rows can be appended at the end of the stream, but existing rows can never be updated or deleted.
Each row is stored in a particular partition. Every row, implicitly or explicitly, has a key that represents its identity. All rows with the same key reside in the same partition.