Ants themselves do not harm your plants, and are sometimes there just to collect nectar. However, ants are little 'farmers' that help several other bad bugs harm you plants. Ants like to farm aphids, white fly, scale, and mealy bug, which then pass 'honeydew', a euphemism for their excrement, which is highly nutritious for the ant colony. The ants stroke the backs of the aphids with their antenna, and the aphids reward the ants with honeydew. As good farmers, the ants protect these bad bugs from their natural predators, attacking ladybug and lacewing larva coming to eat the aphids.
Ants also carry aphids to the tender and most succulent new growth of plants.
To control ants, spray them with Insecticidal Soap. Spray the bugs they are farming with one of the above controls. Follow the ant trails as far as you can to discourage them from returning.