OpenAlias is an alasing system designed to map a domain name to a cryptocurrency address, so that it becomes possible to use those user friendly domain names as addresses.
The basic idea is that domain names are easy for people to remember and map to a particular person/entity. A DNS TXT record (a particular type of data attached to the registration data for a domain) is used to advertise a set of cryptocurrency addresses. A wallet implementing OpenAlias (eg, Monero's simplewallet) will lookup that domain name, find the TXT record, and see the Monero address in it.
The advantages of the DNS route are:
- more user friendly than raw addresses
- registration does not need a blockchain transaction like, for instance, Boolberry's alias system
- prevents "spammy" registration (ie, someone goes and register every single first name)
- support for DNSSEC for authentication and privacy
- an alias can be discontinued or changed
- cross blockchain (ie, you can set up a TXT record with both monero and bitcoin addresses)
Currently, OpenAlias is implemented for Monero (it was originally designed by the Monero team) and Bitcoin (in Electrum). However, it is not intrisically limited to those and could be easily implemented by other currencies.
If you do not have a domain, and cannot add a TXT record to a domain directly, you can apply to an OA service, which allocate you a hostname and associated TXT record. There were two in the past (xmr.link and another I can't recall), but I think they're both inoperative. Maybe a business opportunity ?
See http://openalias.org/ for details.