Ants and aphids have a relationship called Mutualism. The relationship serves to benefit both the ant and the aphids. The ant obtains a sweet source of food from the abdomen called honeydew. The aphids are protected on the branch by the ant. Each can exist independently of the other. But, both parties benefit when together. Here is a link to a better close-up view of ants herding their aphids together. The link also talks about the mortal enemy of the aphid, the Ladybug or Ladybird beetle.
Finally, this video shows some ants tending their aphids. Watch for the stroking action of the antennae on the abdomen of the aphids.
Have you seen other examples of mutualism in your backyard?