One of the questions we get regularly asked is “how come my post looks wrong and the fonts are screwed up” or something similar. Our response is usually to ask whether or not you cut and paste from Word or another word processor.
This is because Word uses a lot of inline style code that when you cut and paste it will follow you to WordPress. If you want to see that, you can click on the HTML tab in editor and you will see lots of <div> and <span> tags.
However help is at hand. Here is a good post on the subject from Rubiqube.