Aphids are very small (only a few millimeters long), soft-bodied insects with usually fairly long antenna. They come in different colours, have small eyes, and sucking mouthparts. They move slowly, and don't jump or hop.
Aphids are herbivores. They suck plant juices out of the leaves, stems, or roots of plants. The juices they drink often have much more sugar than protein. Aphids have to drink so much sugary juice to be able to build their own protein that they excrete a lot of the sugar. They don't need it. The sugary fluid they excrete is called "honeydew", and many other insects feed on it, e.g. ants. They also protect the aphids, and sometimes even keep them in their nests for the winter and put them on new plants in the spring.
Unfortunately, aphids are also one of the worst groups of pests on plants. They damage plants directly by feeding on them, and they carry plant diseases from plant to plant. There can be millions and millions of aphids in a farmer's fields which can cause a lot of damage to his crops.