Cover crops are
plants grown as a living mulch, esp under fruit
trees in home garden orchards. If they are legumes as well they have the added benefit of fixing nitrogen into the soil, as John pointed out.
Green manure crops are the ones you grow to the point of being lush, then till back into the soil, preferably just before flowering. I have used old birdseed for this, oats are great, any out of date
seeds etc as even though not necessarily a legume, the young lush
plants will still add organic matter to the soil when tilled back in. So even if the wild peas are a
weed, as long as you don't let them
flower and
seed, they will help. If they happen to be a legume then that's a bonus.