"Reap" can also be used in a metaphorical sense; to reap the benefits of something = to benefit from something to reap what you sow = to be responsible for your mistakes Both more or less mean the same thing.

Harvest can be used as a noun: "the corn harvest was good..." As verbs they mean the same thing; but reap is an old word not often used now.

'harvest' could also have a literal meaning . There are pretty much the same in meaning,but are used in different contexts and aren't always interchangeable.

another common enough phrase is 'to reap the whirlwind'

To reap is to cut (grain) as with a sickle - there is also 'the grim reaper' as a symbol of death. Harvest is a more general word that includes reaping of grain, but also gathering or catching of other goods like fish or animals.