I took a typing class in high school because my dad said that it would come in handy in college when I started typing papers. This was years before computer were prevalent and I learned on an electric typewriter. Now that I use a computer everyday I admit that it was one of the best things I ever did. In that computer class, I learned that one always types two spaces after a [BrE: full stop/AmE: period]. I used that rule nearly all my typing life.
Then I started blogging and I noticed that WordPress formatted a sentence strangely if I used two spaces after a full stop. If the sentence ended on one line, it started the next line with a space before beginning the next sentence. Well, I hated the look of that so I trained myself to only type one space and that carried over into other things I typed and I use only one space. Turns out, that is the correct way now! Farhad Manjoo of the Slate.com explains:
When I pointed out that they were doing it wrong—that, in fact, the correct way to end a sentence is with a period followed by a single, proud, beautiful space—the table balked. “Who says two spaces is wrong?” they wanted to know.
Typographers, that’s who. The people who study and design the typewritten word decided long ago that we should use one space, not two, between sentences.