Aphids. They are especially attracted to plants fed with high nitrogen. Aphids are THE most common garden pest but generally easy to control. They can be hosed off the plants with plain water for the most part. Many just wipe them off with their fingers.

Otherwise there are any number of pesticides you can use on them if you have a really bad infestation and don't mind the side effects of pesticides. Neem can work but takes some time and can also kill the beneficials that control the aphid population. Just search 'aphid control' here for hundreds of discussions about them.

Dave