If she is usually so cold to you and sometimes becomes so nice(sweet?) such as Suzumiya Haruhi, she would be labelled as ツンデレ. But what if she is so cool rather than cold? And what if such an emotionless girl fawns upon you when she bursts into tears? Yes, she would be a type of クーデレ. クー[coo] derives from cool, and デレ[dere] stems from デレデレ[dere dere] which has a nuance of being so sweet. Speaking of クーデレ girls, I can think of Rei Ayanami[Evangelion], Yuina Himoo[Toki meki memorial], or Mai Kawasumi[Kanon]. If you look at any bishoujo anime carefully, you might find this type of girl.
Here’s one more, ウンデレ[un-dere]. Though I can’t think of any example, ウン[un] is a casual form of “yes.” I think “yeah” or ”yep” would be the closet. To get to the point, this type of girl usually agrees with whatever you say by saying ウン！ウン！Perhaps, she might say like そーだよね～[so-dayone～, "yeah right!"] in order to get much intimacy as much as she could. In other words, she might like you from the beginning. If that’s the case, I could think of Hikari Hinomoto[Toki meki memorial 2] who is a protagonist’s childhood mate.
Personally, I don’t dislike ウンデレ, but I prefer クーデレ or ツンデレ because the more difficult she is, the more I want to pursue her! Also, which do I like クーデレ or ツンデレ? I like ツンデレ because I want her to speak more. Since she has to be cool, a クーデレ girl usually shuts her mouth up, doesn’t she?