Nucleophiles tend to be ions like the chloride ion or molecules like ammonia, while electrophiles are ions like the hydrogen ion or acids like HCl. In general, nucleophiles have more electrons than expected, while electrophiles have fewer electrons than expected.
Nucleophiles are molecules or ions that donate electrons. This is why the ammonia molecule NH3 is a nucleophile. The nitrogen ion in the molecule has a lone pair of electrons that are not tied up in any covalent bonds. As a result, these types of electrons can be donated to form a new bond with another atom. Negatively charged ions are nucleophiles because the electrons that give the ion a negative charge can easily combine with another atom or molecule to form a covalent bond. The other term for a nucleophile is a Lewis base, which is helpful when considering acid-base reactions.
Electrophiles are molecules or ions that can accept electrons. These tend to be positive ions, such as the H+ ion, mentioned above, or the NO+ ion. However, neutral molecules such as Cl2 and Br2 can also be electrophiles. Electrophilic molecules tend to have double bonds because gaining an additional pair of electrons can split the double bond into a single bond.