Spider mites are arachnids with four pairs of legs, that produce silk webs. There are many species of spider mites, among which the European red mite, Panonychus ulmi, is often found on apple trees, and the southern red mite, Oligonychus ilicis, that makes home on many different plants.
Spray the mites with water, so that the water blast removes them from your plants. Most of the mites can be knocked off by the plant by using this simple method.
Spray with insecticidal soap or horticultural oil. These can be found at many home and garden stores, and some even target specifically the red spider mite.
The most environmentally friendly step is to use predatory mites that can consume many of the red spider mites. Also these work with light infestations and when you aren't using insecticides.