I know that alleles are the diff.form of Genes and determine Genes.can anyone better explain what alleles are?

For each gene we have two alleles, one from our mother and one from our father. The nature of the alleles determines  the characteristics of the genes, for example if you have two dominant alleles for a gene then the characteristic seen in that person would confer to the characteristic determined by the dominant allele.

With regards to disease, they are usually caused by recessive alleles so if your parents are a carrier for a disease (e.g. sickle cell anaemia) and they each pass on to you a recessive allele then you will be a sufferer of sickle cell anemia.

An example would be dominant and recessive genes. Two genes may code for the same trait but, depending on which one is dominant/recessive in a  hetero or homozygous individual, two different phenotypes may result.

I think of an allele as a VERSION of a gene....that is in general you will have 2 types or versions of a gene like presence of freckles or absence of freckles, dimple present or dimple absent.  We generally refer to these as dominant or recessive alleles.

I generally say to that recessive alleles are the absence of something eg an absence of a pigment, absence of a step in producing something.....this is why many genentic conditions are recessive eg cyctic fibrosis, albinism, haemophilia, red green colourblindness, etc.....when there is a dominant allele present, the dominant condition is able to mask the recessive condition by ( in general) producing the things that is missing in the recessive allele.

I am not sure what you mean about the chromosomal disorder ....but a chromosomal disorder I guess could be something like a disease like those listed above which is a change in the DNA code which causes these....in fact the difference between a dominant and recessive allele is a change in the DNA code...which one coding for one thing and the other coding fopr something different ( eg forming a protein like colouring for hair, eyes, etc)
Other chromosomal disorders are where some chromosomes have extra copies eg trisomy. This generally only occurs on a few chromosomes as the others will be fatal. eg down's syndrome ( triosomy or chromosome 21) Edwards syndrome, Patau Syndrome.....also Klienfelters Syndrome ( egXXY) . Turner's Syndrome (XO)