I spent a lot of time trying to understand this and thought other people might enjoy it:
1, Higher number of points obtained in the matches played between the teams in question;
2, Superior goal difference resulting from the matches played between the teams in question (if more than two teams finish equal on points);
3, Higher number of goals scored in the matches played between the teams in question (if more than two teams finish equal on points);
4, If, after having applied criteria 1 to 3, two teams still have an equal ranking, criteria 1 to 3 are reapplied exclusively to the matches between the two teams in question to determine the final rankings of the two teams. If this procedure does not lead to a decision, the following criteria 5 to 9 apply in the order given:
5, superior goal difference in all group matches;
6, higher number of goals scored in all group matches;
7, position in the Uefa national team coefficient ranking system;
8, fair play conduct of the teams (final tournament);
9, drawing of lots.
( Explanation goes hereCollapse )
EDIT ONE THING: Guys, these are the current rules. Article 8.07 of the EURO 2012 regulation pdf (approved in 2010) doesn't list criteria #4, but if you scroll all the way down there's an ammendment from March 2012, to come into effect immediately, adding it ;) Hope that cleared it!