 Secondary structure of proteins refers to the local confirmation of some part of a polypeptide chain. So, if we have a polypeptide chain and that polypeptide chain locally makes some change in its confirmation, that can be called as the secondary structure. For example, here you can see this is a polypeptide chain. It contains number of amino acids. It is fully extended polypeptide chain and it can make quail or helix. It can make a sheet or it can make a turn. It can make a loop. So, all these changes are local changes, local confirmational changes and all these changes are called secondary structures of the proteins. For example, it includes a number of structures like helix, alpha helix, beta sheets. Turns include beta turns and gamma turns. It contains loops like omega loops. It also includes random quails. So, there are number of secondary structures which are repeated in the structure of protein and all these are local confirmational changes in the structure of a polypeptide chain. However, a few type of secondary structures are particularly stable and occur widely in the proteins and these types include alpha helix and beta confirmations. In alpha helix, a polypeptide chain is helically coiled round an imaginary axis and it makes alpha helix. This is very prominent in the proteins. The second most prominent structure is beta confirmations. In the beta confirmations, polypeptide chains, they stretch side by side longitudinally and they form sheets and these sheets are called beta sheets. So, alpha helices and beta sheets are the prominent secondary structures found in different protein molecules.