Fel Fashion is a blog dedicated to World of Warcraft transmogification. Transmogging is the ability to change your armor to look like another piece, with some restrictions –

• Transmog is only allowed on like armor types – cloth can only be transmogged to cloth; leather can only be transmogged to leather; etc.
• Transmog does not change the stats on an item.
• Transmog is not permanent and can be redone at any time.
• Legendary items cannot be transmogged at this time.
• Transmogs are allowed on items with the same animations – so 1h swords, maces, and axes can all be interchangeably transmogged. Polearms and staves can also be transmogged. *
• * You have to be able to use a particular weapon type to be able to cross-transmog it. For example, a warlock cannot transmog staves <-> polearms, because warlocks cannot use polearms.
• Items in your bank, void storage, or bags can be used for transmog.

I hope this helps clarify some questions you may have about transmogging.