Colons can be used to punctuate sentences where the second part explains or illustrates the first part.

It wasn't easy: the questions were all at level 5.

In this example, using two separate sentences or a conjunction would also be correct.

It wasn't easy. The questions were all at level 5.

It wasn't easy because the questions were all at level 5.

However, using a comma to join the clauses would not be correct.

It wasn't easy, the questions were all at level 5. (wrong)

Using colons in this way is also a way of emphasising a point.

Sid is particularly well known for his greed.

By rewriting this sentence with a colon, Sid's greed can be highlighted.

Sid is particularly well known for one thing: his greed.