If you use variable width fonts, such as Arial, Helvetica, Times New Roman, etc. then the software works in the space required to separate a punctuation mark and the next letter. Fixed width fonts (such as those used in typewriters) don't give you that luxury, so it is custom to use a double space (end of sentence) or single space (other punctuation) to differentiate.
Have a read of "The Mac Is Not A Typewriter" (or the PC version) for more formatting tips.