Bitcoin is not anonymous, but, rather, pseudo-anonymous. By now, most Bitcoin veterans know this. It’s less obvious to many, however, why Bitcoin is not really anonymous by default, and what can be done to de-anonymize Bitcoin users – and what Bitcoin users can do to reclaim their privacy. However, There are generally three reasons why bitcoin is sometimes regarded as anonymous.

First, unlike bank accounts and most other payment systems, Bitcoin addresses are not tied to the identity of users on a protocol level. Anyone can create a new and completely random Bitcoin address (and the associated private key) at any time, without the need to submit any personal information to anyone.

Second, transactions are not tied to the identity of users either. As such, (and as long as a miner includes the transaction in a block) anyone can effectively transfer bitcoin from any address to which it controls the (private) keys, to any other address, with no need to reveal any personal information at all. Like physical cash, not even the receiver needs to know the identity of the sender.

And third, Bitcoin transaction data is transmitted and forwarded by nodes to a random set of nodes on the peer-to-peer network. While Bitcoin nodes do connect to each other using IP-addresses, it's not necessarily clear for nodes whether the transaction data they received was created by the node they connect to, or if that node merely forwarded that data.