In the US PhD prospects apply to several universities and those select an amount of applicants from that pool according to how many open spots the universities have for that term. Criteria for admission are GPA, GRE/SAT and research experience documented by 2-3 letters of recommendation.
In Europe the application process is often times quite different in the way, that not a selected number of applicants get admitted each year, but each available research/teaching assistants position is treated individually. Because the requirements differ vastly, a lot of the times the only formal requirement for application is a 'good' or 'very good' masters degree and/or experience in field x or with technology y.
I wonder, are letters of recommendation expected, if so how many, and how important are they actually?