Sometimes there is no real difference - eg, fruit which is kept by being bottled in sugar (jams etc) can be called both conserves and preserves.
Where there is a difference, conserve means to keep, as in not use up, a resource or material. You conserve your energy in a long-distance race, rather than using it all at the start. A conservationist tries to prevent waste or over-use of natural resources - trees, fish stocks etc.
To preserve usually means to keep or rescue from harm - eg a city could be preserved from invaders. It also means to keep something from decaying after death - for instance, embalming is a way of preserving dead bodies. Thus, a preservative is a substance, such as vinegar, which can be used to prevent foodstuffs from going bad.
So, broadly speaking, you might conserve an area where rare wild flowers grow, in order to preserve the flowers from harm.
