Mappings mapping(key_type => value_type) [public] name;

mapping (address => int) funds;

funds[address] = 33;

- set value

funds[address];

- retrieve value

Simple types can be used as keys. Any types can be used as values.

All possible mapping keys always exists and have a default byte value of all zeroes.

Can't be iterated, checked for length, retrieve which keys were set etc. Also can't be created in memory.