This lets you speak a name to dial a number instead of entering a number manually or choosing it from the phone book. Most phones with this feature limit the number of voice dial entries to a small number - a fraction of the size of the phone book - although some allow the use of shared memory, which usually allows a large number of voice dial entries.
There are two types: speaker-dependent (recorded) and speaker-independent (recognition).
With speaker-dependent, voice dial entries must be explicitly created by speaking and recording the name 1-3 times. The phone will only respond to recorded names, and usually only when spoken by the same person who recorded them.
With speaker-independent voice recognition, no recording is required. The name can be spoken by anyone, and the phone will automatically match the spoken name with the closest name entered in the phone book.