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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.
