Beta (uppercase/lowercase Β β), is a letter of the Greek alphabet, used to represent the "b" sound in Ancient Greek and "v" in Modern Greek. In the system of Greek numerals, it has a value of 2. Letters that came from it include the Roman B and Cyrillic Б.
The sounds [b] and [v] resemble each other a lot. In Ancient Greek the "b" sound became "v" around 200 BC. Therefore, the letter B means the "v" sound in the Cyrillic alphabet, and a new letter, Б, was made for [b].