In a preimage attack, the attacker starts with a particular output, and is able to find an input that will produce that particular output, whereas a collision attack merely finds two inputs that produce the same output.
The attacker starts with a particular output. This is in a preimage attack. The attacker is able to find an input. This is in a preimage attack. This input will produce that particular output. A collision attack merely finds two inputs. These two inputs produce the same output.