Chromosomes and exons - (Nov/14/2011 )

Hi:
I have a newbie question, actually I want to make comparisons between full chromosomes. I have found that in ftp.ncbi.nih.gov/genomes/ there is a chromosomes list, but which one is the correct to download? I need the fasta files, for example for chromosome 2 it would be fine if I download the one that says hs_alt_HuRef_chr2.fa.gz? what does hs_alt stands for?
Also I suppose that if it is the complete sequence it contains the introns and exons from each chromosome, is there any site where I can download only the exons or the introns of each chromosome?
Thanks for your replies

I don;t know if Ensemble works. But you can try it. Good luck!