ISRC is the International Standard Recording Code – it’s a unique identifier for every recording you make.  It’s unique to an artist, and the country of origin, and the year released.  You don’t need them, per se, but if you’ve got them, great.  They’re useful for tracking songs for copyright and statistical reasons.  Say your recording appears on 5 compilations – the ISRC code would stay the same across all of them, amkign it possible to track sales of that one particular song.  This is particularly useful in Europe, where this is tracked well (less so in the US).  If you want an ISRC code, you can usually get them assigned to you by a label or a distributor, but if you want to assign your own, there’s a $80 one-time fee (in the US) from http://www.usisrc.org/.  If you want to assign them to other artists, you need to pay an annual fee.  This varies from country to country, so if you live in Andorra, I’m not sure who you need to go through.