Baa is the sixth letter of the Samaritan alphabet and carries the numeral value six. It stands where Hebrew has vav, and its name records the fact that the Samaritan reading tradition sounds this letter as b rather than as w, which is why the standard character name for it is Baa and not Waw.
By any measure of use it is one of the two or three most important letters in the script. Prefixed to a word it is the conjunction and, and Hebrew narrative prose is built out of clauses strung together with it, so the letter appears at the head of an enormous proportion of the sentences in the Torah. Inside a word it does duty as a vowel letter for the u and o vowels, which is a second job entirely, and one that unpointed Semitic writing depends on to be readable.
Greek used this sign twice over. It kept it in sixth place as digamma, a letter for the w sound that dropped out of the Greek alphabet as the sound itself disappeared from the language, though it survived as a numeral. And it took the sign again, reshaped, and placed it at the end of the sequence as upsilon, a vowel. The Latin letters F, U, V, W and Y all trace back through one or other of those two branches, so a single Samaritan letter stands at the head of five modern ones.
The character is encoded at U+0805.