An adaptation is a change in an individual in order to fill a niche or survive in its environment. Adaptations add up over time to drive Natural Selection.
Adaptations are usually favorable, but sometimes adaptations can be less than ideal. The favorable adaptations allow the individuals to live long enough to reproduce and pass down those adaptations to the next generation. Adaptations that are not favorable will eventually be bred out of the population. That is essentially the definition of Natural Selection.
Adaptations can be on a molecular level, like a change in DNA, or they can be behavioral.
A behavioral adaptation is phototropism - when a plant bends toward a light source.
A structural adaptation, encoded by DNA, would be amount of skin pigmentation. Humans who live closer to the equator in direct sunlight tend to have a darker skin color than humans who live in colder climates.
Another example of adaptations can be seen in bacteria colonies and their antibody resistance. Bacteria that have adapted to be immune to the effects of the antibody will live and reproduce, while the weaker strain that was not immune will die off. This leaves only bacteria that are immune to the antibody so the the antibiotics no longer work to kill off the bacteria.