helix
The helix is one of three types of secondary structures in Foldit. The other types are sheet and loop.
Some times the prefix "alpha" is added, so you might see "alpha helix" or "α-helix". The "alpha" reflects the fact that helices were the first type of protein structure discovered.
In its ideal form, a helix appears like a coiled cylindrical spring with all sidechains directed outward. Ideal helices usually have hydrogen bonds between every third amino acid in the helix.
The Foldit view options "Show Bonds" (basic GUI) or "Show Bonds (helix" (advanced GUI) can be used to visualize helix bonds. In the advanced GUI, the "cartoon" or "cartoon thin" protein view options show helices with the coiled shape seen in the example on this page.
An ideal helix has one turn of the coil for every 3.6 amino acids.
Certain amino acids have a tendency to form helices, while others tend not to. Those that often (but not always) form helices are methionine, alanine, leucine, uncharged glutamate, and lysine ("MALEK" in the amino-acid 1-letter codes). Those that do not easily form helices include proline, glycine, and aspartate. Proline, however, may be found at the beginning of a helix.
Helices as Part of Larger Structures
Editor's note: this section is interesting, but it's perhaps a bit on the technical side for most.
Coiled-coil α helices are highly stable forms in which two or more helices wrap around each other in a "supercoil" structure. Coiled coils contain a highly characteristic sequence motif known as a heptad repeat, in which the motif repeats itself every seven residues along the sequence. The first and especially the fourth residues (known as the a and d positions) are almost always hydrophobic (the fourth residue is typically leucine) and pack together in the interior of the helix bundle. In general, the fifth and seventh residues (the e and g positions) have opposing charges and form a salt bridge stabilized by electrostatic interactions. Fibrous proteins such as keratin and myosin often adopt coiled-coil structures, as do several dimerizing proteins. A pair of coiled-coils - a four-helix bundle - is a very common structural motif in proteins. For example, it occurs in human growth hormone and several varieties of cytochrome. The Rop protein, which promotes plasmid replication in bacteria, is an interesting case in which a single polypeptide forms a coiled-coil and two monomers assemble to form a four-helix bundle.