The term or terms that are within your keyword list are the terms that trigger your ad. Just be mindful of the match types that you are bidding on and to place in your search term correctly when searching. I have placed an example below:
Broad: red Search Term: I want a red flower
Phrase: red Search Term: red flowers or flowers red
Exact: red Search Term: red
By searching for your keywords, please search according to the match types bidded upon so that you can locate the ad with ease and high relevance.
Also there is a "Research Tab" within the account and this will supply you with alternative keywords that are being searched on the Live.com site.
so in other words the ONLY way to advertise on adcenter without being screwed is exact match only. Use 3rd party sites such as wordtracker to build your list prior to putting one red cent on MS's table.
With MSN & Yahoo I'd stick to exact match and nothing else. Both are light years behind Google, in every way you can think of. Limit the money you give them and they might get their arses into gear - or they can contact somebody like myself who has around 100 'no brainer' suggestions which they should have implemented from day dot!