a gene is a segment of DNA which exists at a particular locus on a chromosome and codes for particular proteins (or just one). As autosomal genes are duplicated (1 maternal and 1 paternal chromosome), you have 2 genes directing the production of each protein(s).
an allele is a variation of the gene. there can be many more than 2 variaties of a gene (alleles) in a population but each individual would only possess either 1 (homozygous; same allele at both loci) or, at most, 2 (heterozygous; different alleles on the maternal and paternal chromosomes) alleles of a gene.
a trait is not always determined by a single gene. Height, for example, is the combined affect of many genes.
hope this helps.