Joshua Brustein writes:
While Bitcoin is regularly described as anonymous, it is more accurate to describe it as pseudonymous. Every transaction is stored in a public record called a block chain, with information on the accounts involved and the number of Bitcoins exchanged. This is a necessary part of the structure; analyzing these transactions is how the system makes sure that people aren’t using the the same coins over and over. While Bitcoin is not as transparent to regulators as credit-card transactions, it is much more so than cash.
Read more here.