(I feel like this should be pinned to the homepage or something, as I've seen this question a LOT.)
Here is a link to their official policy statement:
"A Trademark must be Protected
A trademark must be able to distinguish the goods of one company from those of another. If a trademark loses this ability, the owner may find that it can no longer prevent others from using the trademark. An owner must prevent the improper use of its trademarks to prevent the public from being deceived. This is why the LEGO Group is very active around the world in making sure that its trademarks are not misused."
"Proper Use of the LEGO Trademark on a Web Site
If the LEGO trademark is used at all, it should always be used as an adjective, not as a noun. For example, say "MODELS BUILT OF LEGO BRICKS". Never say "MODELS BUILT OF LEGOs". Also, the trademark should appear in the same typeface as the surrounding text and should not be isolated or set apart from the surrounding text. In other words, the trademarks should not be emphasized or highlighted. Finally, the LEGO trademark should always appear with a ® symbol each time it is used."