Alleles are the different forms that the same gene can take. Human beings have duplicated genetic material; we have received one copy from our father and another copy from our mother. Therefore, we also have genes in duplicate, a maternal copy and a paternal copy. Each of these copies is an allele.
These two alleles, which belong to the same gene, can be the same (same DNA sequence), and the individual is considered homozygous for that gene, or different (the DNA sequence is different between them, there are differences in their nucleotides), and the individual would be heterozygous for the gene. In all cases, alleles of the same gene carry information for the same protein or regulation, but the result can be different. Some alleles carry non-productive information, being unable to encode their protein or being more inefficient than the normal form, and causing diseases.