core input formats
Site configuration > Input formats
Watch out for these. Anything wrong with your content formatting, check here first.
Issues
For reasons that I still don't understand. at times the default input format, usually filtered html, switches to another input format. Then you wonder what happened to your formatting.
Also be carefull experimenting with an input format. The experiment will have immediate effect on all content. Luckily it only effects the way content is shown, not the way it was originally formatted. That means any changes in the way your content looks is reversible. You just need to change it back to what it was before.