Huckel's rule is a fairly straightforward principle. Basically it states that if a compound possesses 4n+2 pi electrons (where n can be an integer greater than or equal to zero--i.e., 0,1,2,...), it is aromatic, PROVIDED that the compound is cyclic, planar, and has an unhybridized p orbital on every atom in the ring structure. For instance, benzene is a six carbon ring structure. Every carbon in the ring is sp2 hybridized, which means that there is an unhybridized p orbital on each carbon. Moreover, the structure is planar. Now you simply apply Huckel's rule as a final criterion for aromaticity. Here's how you apply it: each end of a double bond equals one pi electron, and lone pairs of electrons may or may not be considered pi electrons. In the case of benzene, there are three double bonds (which can be shifted around as per resonance), and therefore there are 6 pi electrons. Can 4n+2 equal six, given the restrictions on the value of n? Definitely. If n equals 1, then 4n+2 equals 6. Oila, benzene satisfies Huckel's rule.
Huckel's rule can become a little bit tricky when cations and anions are being evaluated for aromaticity. Solid working knowledge of hybridization will help you tackle problems of this sort.
Hope this helps.
