Changes in the composition or structure of a protein often occur after translation. More often than not, these modifications are necessary for that protein to function properly. A lot of them are reversible.
Types of PTM
There are many types of post-translational modification. These are just a few of the common ones.
Basics
Enzyme catalyzed reactions are usually the source of modifications. Each amino acid in a protein has certain modifications that it is prone to. Only cysteine can form disulfide bridges. Not all modifications involve changes to the side chains of amino acids. Peptidases, enzymes that cut other proteins in half, attack at points with a specific amino acid sequence. Many modifications happen at the C or N terminus of a protein.
Significance
It is pretty easy to express any protein, but ensuring that it has the proper modifications can be extremely difficult. As stated before, most proteins will not do their job if they do not have the proper modifications. This is true of miraculin and conotoxins. Many modifications are used for both intracellular and intercellular signaling. Proteolytic cleavage performed by a signal peptidase is used to remove the signal peptide from a protein that will reside in the membrane or be secreted from the cell. Other modifications, like phosphorylation, are used to store energy. Conventional proteomics may not provide as much useful data as its advocates claim because modifications are not detected. Unwanted modifications may render a protein useless or cause it to malfunction.
Web Reference
http://www.indstate.edu/thcme/mwking/protein-modifications.html
http://us.expasy.org/tools/findmod/