Nucleophiles and electrophiles are important classes of molecules as they are involved with a large number of organic reactions. Success and happiness in organic chemistry relies on you being able to identify which is which.
Nucleophile means “nucleus lover” and is pretty much what it says on the tin – these are c0mpounds that are strongly attracted to positive charge. It follows that they themselves are electron rich, sometimes with a formal negative charge. Good nucleophiles are also very reactive. You know of another class of compounds that are reactive and electron rich – bases. Nucleophiles are generally strong bases, or the conjugate base of a weak acid.
Electrophiles are the counterpart to nucleophiles. They crave electrons as they are electron poor, (sometimes positively charged). You won’t see the term “electrophile” as much as you will “nucleophile”, but you will see “electrophilic site”, which refers to the reactive, electron poor part of the substrate.
Leaving groups are the part of the molecule that is booted off during a substitution or elimination reaction. They must be stable on their own, and not too tightly bound to the substrate. They are generally negatively charged, as they take the electrons with them when they leave. A good leaving group is a weak base, or a conjugate base of a strong acid. Note the difference between a g0od nucleophile and a good leaving group. This is an important distinction.
See? I told you, you need to know about acids and bases!