Aphids, also called plant lice, are a tiny, sapsucking insect. They are attracted to fresh new shoots of plants, especially developing fruit and flowerbuds. They can do a lot of damage to crops, and gardeners do not like them. Aphids can be killed by poison, removed by squirting them off with water from a hose, or their numbers can be reduced naturally by encouraging aphid-eaters like ladybugs and wasps into your garden.
The aphids in the picture are on a the developing flower bud of a rose.