Covalent bonds are generally the strongest chemical bonds. In Foldit, the peptide bonds that hold the protein's backbone together are a type of covalent bond.
A disulfide bridge is another type of covalent bond found in Foldit. Unlike peptide bonds, you can make and break disulfide bridges as you fold the protein.
The hydrogen bond is other main type of chemical bond in Foldit. Hydrogen bonds are much weaker than covalent bonds.
For more information, see the wikipedia article "Covalent bond".