When registering I am asked to enter a Bitcoin address. What is that?
A Bitcoin address is your ID (account, wallet number), starting with 1 or 3 and containing 27-34 alphanumeric Latin characters (other than 0, O, I). The address can also be represented as a QR-code; it is anonymous and does not contain information about the owner. An example would be: 3MZiY9v4PesZ2Ek9iYD56bRZtrvNZm99AL or 3Bby7QXjWBbz1y96hX4NjjrGcNT7VjFqRi.