Upper- and lower-case beta.
The Greek letter beta (Β or β) is used in biochemistry, and sometimes finds its way into Foldit.
The terms "beta sheet", "beta strand", "β-sheet", and "β-strand" all mean sheet. The "beta" prefix reflects the fact that sheet was the secondary structure type identified, after helix.
The beta carbon is the first sidechain atom in every amino acid (except glycine, which doesn't have a sidechain).
A capital beta normally looks just like a capital Roman "B". A lower-case beta usually looks like a smaller version an upper case Roman "B", with the stroke on the left descending below the line.