The tertiary structure is the third structural level of proteins. Remember, the primary structure is when amino acids (subunits of polypeptide chains) are arranged in a polypeptide chain held together by peptide bonds. The secondary structure is when the chains bend to form shapes such as an alpha helix or a beta pleated sheet and involve hydrogen bonds. The tertiary structure involves multiple coils and pleated sheets combined with each other creating a more complex structure where the alpha helix and beta pleated sheets fold in on themselves. The multiple polypeptide chains are held together by disulfide bonds.
Source: Concepts in Biology. 12th edition. Copywright 2007 McGraw-Hill.