techno is a term used to describe all or most electronic dance music to people that don't know a whole lot about it
electronica seems to be interchangable with the word techno in this sense
techno itself however, is also a specific genre of electronic dance music, similar, but much different than "house, "trance", "drum and bass", "hardcore" and other genres of electronic dance music
here is a good way to help identify different forms of electronic dance music and to get some good examples of various genres:
http://www.ishkur.com/music/