Denotation is the literal meaning of a word, and connotation is what tone it implies.
Denotation is the dictionary definition of a word, the surface meaning. Connotation is the contextual definition, the tone it is used with or maybe the reaction that it prompts. For example, sly and clever have basically the same denotation, they both mean crafty and fast thinking. However, sly has a negative connotation, being used to describe someone who is deceitful or has bad intentions. Clever, on the other hand, has a positive connotation, probably used when speaking or writing of someone who is intelligent, or is quick to learn and think.