Amides are made by a chain or carbon atoms with one as a carbonyl, double bonded to oxygen, bonded to an amine group, a Nitrogen, which is then bonded to the next carbon to continue the chain and one hydrogen. The carbonyl group must be adjaent to the amine group to make it an amide. Since they form chains, they can continue for a long number of atoms, this allows for one chain to have many amide groups in one chain.
This ability is what allows it to take the alpha helix and beta sheet forms. The hydrogen on the amine can form a strong hydrogen with the lone pairs on a carbonyl oxygen further down the chain. A large number of these hydrogen bonds makes the new structure very strong and makes the chain coil round into the alpha helix structure with another chain, as we see in DNA.
The beta pleated sheet structure, also found in DNA, is formed by a series of chains laid out over each other. The Hydrogen bonds form then between the sheets, forming again, a strong structre that is hard to separate.
These then form a larger structure by chains, alpha helixes and beta pleated sheets all bonding together and coiling or bending over each other, making a tertiary structure, held together by intermolecular forces. These tertiary structures can then bond together to make quaternary structures.