SAM is the current standard format for storing aligned sequences. It was defined by the community in the early days of the 1000 genomes project.

SAM format tutorial

Samtools commands tutorial

BAM is the compressed version of SAM.

However the newest format is CRAM. A CRAM file is about 30% the size of a BAM file. CRAM is now being adopted by important bioinformatics tools like Samtools.

Size, why are SAM /BAM files so big?

Roughly speaking, a SAM file is about 1 byte per base, a BAM file is about 4 bits per base, and a CRAM file is 1-2 bits per base.

The human genome has about 3.3 billion base pairs, meaning that whole genome sequencing at only 10x would result in a SAM file with 33 bilion bytes of data (aka 33Gb of data). Therefore a 30X whole genome sequencing would produce a SAM file of 90Gbs at a minimum. HOWEVER, that is taking into account the genome sequence only. SAM files also contain quality scores for each base call, cigar strings, and etc.