Gene Mutation.

Mutation: Change in quantity or structure or DNA of an organism.

Nonsense mutation:
The mutation creates a stop codon. I.e. The mutation changing one base. Say the recognised stop codon is AUG. And, currently, your codon is ACG. If the C was swapped for a U, this would now be a stop codon, do the polypeptide chain would stop being produced at that point. Final protein would be different, and nto function the same.

Mis-sense Mutation: 
If the codon ends up coding for a different amino acid. Say ACC codes for Alanine, and ACG codes for
Glutamine. If the final C on the initial codon was swapped for a G, the codon would now produce Glutamine, not Alanine.The amino acids determine the tertiary structure - so the shape of the protein is likely to be changed, possibly making it non-fucntional.

Silent Mutation:
The Mutation changes a bases, but the codon still does for the same amino acid as previously. This is due to the genetic code being DEGENERATE. 

These are all SUBSTITUTION

Deletion of Bases:
A single base is deleted from the code. Although initially thought to be a mild effect, this usually causes what is known as a frame shift. Meaning - where the one base has been removed, the entire chain left shifts over by one place to the left. This changes all of the codons after that point, so the nearer to the begining of the chain, the worse off it is.

Genetic mutation is RANDOM. Mutagentic factors can affect this however. So, high energy radiation, and chemicals that alters the DNA structure, or interferes with transcription.