The Colon
:

The colon can be used to introduce: a list, explanation or definition.

Code:
WF.com improves many things: your writing, your reading and your sex drive.
--
There's only one thing I want: to dance!
--
Write: (verb) To spill your guts for an unappreciative public.
A colon will always follow an independant clause. If you're wondering whether or not to use a colon, try taking the first clause of your sentence (before the colon) independantly. It might feel incomplete by itself, but grammatically should nonetheless have a subject and predicate. You cannot follow a dependant clause with a colon.

The Semi-colon
;

The semi-colon is primarily used to join two independant clauses. It allows us as authors to imply a relationship between two similar ideas without actually stating that relationship.

Code:
I hate maths; she hates art.
The semi-colon is also commonly used to join clauses via a transition.

Code:
WF.com is a great place; in addition, the members are gorgeous.
I love sea-food; therefore, I shall order prawns.
Love is tiring; however, it's good for the heart.
You might also want to use semi-colons to order a list as below:

Code:
WF.com has many kind members with charming interests: Dark Aevin, sadism; daniela, The Doors; eleutheromaniac, the Red Sox; Penelope, history; Farror, squash; Hollyoake, Garfield...