Old threads with the same question (Shameless link-ins to my own posts):
Namecoin - what is going on?January 06, 2012, 12:23:41 AM
https://bitcointalk.org/index.php?topic=56486.msg677176#msg677176What's going on with Namecoin?April 16, 2012, 03:09:52 PM
https://bitcointalk.org/index.php?topic=76320.msg854222#msg854222There was one proof-of-concept for using Namecoin to store an alias (like a nickname for your bitcoin address), but it never was really live. Otherwise, it is a DNS system that nobody uses for resolving domains. It is still 0.3.24-based, so any security concerns that have been addressed in new Bitcoin updates are still present. It works fine and doesn't need shameless pump-and-dump promotion, since it was the first alt-coin and wasn't created by some get-rich speculator.
If you have some interesting project that needs (name)->(some number) mapping or have some other concept that could use it, it will keep on making a blockchain for you to store your data in.