I am a firm believer in creating prose that is as lean as possible. A paragraph that doesn't develop the plot, setting, or characters is a paragraph wasted. Even sentence-by-sentence there needs to be a constant flow of focus.
My least favorite form of over-description is excessive adjectives. There are very few places in fiction written for adults where listing more than two adjectives in a row is acceptable. In children's there are more, but it's still something to be avoided in most character voices. If you can't describe an object well enough for the story in two adjectives, you may actually be under-describing. Try having one or more characters interact with that object and let your readers learn that way.
On a related note, too many adverbs can be either over-description or under-description. If the way a character does something doesn't matter, don't include an adverb, please. If it does, try to use concrete description, not adverbs, to display how the action is being performed. Rather than "Ron moved swiftly to the car," you can use "Ron moved to the car," "Ron sprinted to the car," or "Ron threw open the door and tore through his front yard, vaulting a hedge in his haste." Adverbs are to be used only as a last resort in prose. Feel free to use as many as you'd like if you write non-fiction as well.
A character's appearance can say a lot about him/her, but so do their actions and others' dialogue. Only describe a character's looks in-depth if it develops plot or setting in an interesting way, unless you cannot possibly explain a character trait by any other method (which is a rare scenario). That is, unless your viewpoint character's personality makes it necessary (like if you're writing in 1st-person about a cosmetologist, for example).
These are three of the biggest ways that writers fall into over-description. Writing more would be over-description (at least for A-to-Z Challenge purposes).