What is the difference between exothermic and endothermic?

Exothermic and Endothermic are opposite reactions. All reactions need energy to be put in to start them off, this is called the activation energy, but the overall energy gain can be either positive or negative.
EXOTHERMIC- These reactions are ones which lose energy. The reactants have more energy than the products, and so energy is given off to the surroundings. A common example of this is combustion, you can tell that energy is lost from the system because the surroundings heat up- if you put your hand over the beaker it feels warm. The overall energy gain of the system is negative.
ENDOTHERMIC- These reactions gain energy from the surroundings, as a result the products have more energy than the reactants. An example of this would be photosynthesis: the plant is taking in light energy to produce glucose- a high energy product. A second example is chemical ice-packs, they cool you down by taking heat energy from your skin to produce products with a higher chemical energy than the reactants. The overall gain of the system is positive.
All reactions are either exothermic or endothermic. The amount of energy lost/gained is all to do with the chemical energy stored in the bonds of the substances.