Sheets are also called "strands", and sometimes the prefix "beta" is added, so you may see beta-sheet or β-sheet, beta strand or β-strand. Technically, a "strand" is a section of the protein, and a sheet is two or more stands bonded together. In Foldit, the term "sheet" is normally used for both sheets and strands.
The "beta" in beta-sheet reflects the fact that sheets were the second type of protein structure discovered, after the "alpha" helix.
The main attribute of sheets is that they readily form multiple hydrogen bonds with neighboring sheets.
The Foldit view options "Show Bonds" (basic GUI) or "Show Bonds (sheet)" (advanced GUI) can be used to visualize the bonds between sheets. In the advanced GUI, the "cartoon" or "cartoon thin" protein view options show sheets with the lightning bolt or zig-zag shape seen in the examples on this page.
Parallel and anti-parallel sheetsEdit
The terms "parallel" and "anti-parallel" are sometimes used with sheets. Two sheets are anti-parallel when the lowest-numbered segment of one sheet is adjacent to the highest-numbered segment of the other sheet. This often happens when one sheet is followed by a short turn, followed by the second sheet.
Two sheets are parallel when the lowest-numbered segment of one sheet is adjacent to the lowest-numbered segment of another sheet.
The hydrogen bonds between anti-parallel sheets tend to all point in the same direction. The hydrogen bonds between parallel sheets tend to point in alternating directions. (So, the bonds between anti-parallel sheets are parallel, and the bonds between parallel sheets are anti-parallel....)
The bonding of anti-parallel sheets is stronger that the bonding of parallel sheets.