What is an allele .... please explain !?

Allele - Any one of two or more alternative forms of a gene that may occur alternatively at a given site on a chromosome. Alleles may occur in pairs, or there may be multiple alleles affecting the expression of a particular trait. If paired alleles are the same, the organism is said to be zygous for that trait; if they are different, the organism is heterozygous. Eg T (tall) or t (short).