Andar sticks to a single meaning (as a verb), which is to walk. Man's locomotion by foot without running nor trying any fancy things. May be fast or slow.
Caminhar is andar with some further information attached. Caminhar can be andar without hurry, a slow and/or calm walk to appreciate the surroundings, to think or to empty one's mind. It can also be a sport walking, in that case it is not slow.
Caminhada (noun) can be walking for sport, hiking and trekking.
Not exactly. "Andar" can be used in other ways: "ando de fiesta" (I'm celebrating or partying), "ando enfadado" (I'm mad), "ando de paso" (I'm passing by), etc. It can have the sense of being involved in an activity or emotion. "Caminar" is to walk, often in a leisurely fashion or for sport. Djeidot indicated a similar distinctionin Portuguese.