A semi-colon can be used to join two clauses instead of a conjunction.

for example:

I love ice cream because it's so cool on a hot day.

I love ice cream; it's so cool on a hot day.

Both these sentences are correct, but using a semi-colon adds variety to writing and makes it more interesting. In a way, the reader understands the word 'because' in the sentence even though it is not actually there.

Note that it is not correct to use a comma to join two clauses in this way. This is a very common mistake.

I love ice cream, it's so cool on a hot day. (wrong)

A semi-colon should only be used if the sections on both sides make sense in their own right. 'I love ice cream' and 'It's so cool on a hot day' can both stand as independent sentences.

If the second part of the sentence adds information to the first part but could not stand on its own as a sentence, then a colon should be used instead.

There is only one flavour of ice cream worth eating: strawberry.