Nucleophiles may be negatively charged and standard A level nucleophiles include OH-, Cl- and CN- but they don't have to be.
H2O and NH3 are also common A level nucleophiles.
If you're studying IB Chemistry then you should know that the ability to donate a lone-pair'makes every nucleophile a Lewis Acid.
If you're studying A' level Chemistry then you can forget about Lewis Acids altogether.
You are most likely to need know about nucleophiles when dealing with nucleophilic substitution.
The details of the above are on another page but it's important to realise that charged nucleophiles (OH-, Cl- and CN-) follow a simpler mechanism than neutral nucleophiles (generally NH3</sub).
You should certainly remember that the lone-pair on a cyanide nucleophile (CN-) is not where you might expect. You almost never see a Nitrogen atom without a lone-pair, you almost never see a lone-pair on a Carbon atom - but that is where it is.