From MIM Wiki
All foreigners intending to stay in Poland longer than 3 months, are requested to obtain a PESEL number (Public Electronic System of Population Records) regardless if the residency in Poland is temporary or permanent. In that case you require also a document allowing you to stay on the Polish territory (i.e. work permit - pozwolenie na prace, residency card - karta pobytu). If you cannot present it to a public officer at this time, it is allowed you confirm by your signature you are in a possession of such a document.
 What is this?
PESEL is a unique 11-digit number. It contains coded date of birth, sex and a specific number of the registered person. It doesn't change during the whole life regardless of the place of residency. You will need the PESEL number for several bureaucratic issues, but also to sign a monthly fee contract with a mobile phone provider.
 How do I get it?
To get a PESEL number you need to fulfill a couple of legal obligations that hang over a foreigner. First thing is obtaining a residence permit for a fixed period of time and the second is registering your place of residence (cf. here).
The PESEL number is given automatically after registering your place of residence and the procedure does not require any filling out of forms or other bureaucratic steps. It takes up to 4 weeks for the City Council to issue you with the PESEL, but it may be less depending how busy the Council is (a good idea would be calling your City Council after 2 weeks to check if they have issued the PESEL for you). You can pick up the document with your PESEL number in person, however you will need to take a document confirming your identity with you.
For more informations, check http://en.wikipedia.org/wiki/PESEL.