They are both the offence of causing 'grievous bodily harm', which is serious injury.
S.18 is the more serious of the two offences, because the person who caused the injury intended to cause very serious injury to the victim. He meant it.
The maximum penalty is life imprisonment.
S.20 is the same amount of injury, but the offender didn't intend to cause that much harm.
A good example is a hard push. The victim falls down a very steep flight of concrete stairs and suffers a broken skull, broken ribs and a broken leg.
For a s.18, the person who pushed was well aware that that the stairs were there and meant for the victim to get seriously hurt.
In s.20 the person who pushed believed there was only one or steps, and the victim would merely stumble