- Pure Ambient - This is what most people think of when they hear the word 'ambient'. There are zero beats here; it's all light synths/keyboards/guitars/basses/noises. The songs just drift along, there is nothing hold them down. This is usually more of a direct attempt at evoking a feeling or a place, without the distractions of overbearing rhythms. Dark ambient and space music can also fit into this category. A good example of this is Seven Ancient Glaciers, by Aglaia.
- Beat Ambient - A silly name, but you get the point. It's still very relaxing, but there is usually something going on that will grab your attention more, whether it be beats, vocals at the forefront, a faster tempo, whatever. This can be closely related to trance, house or folk, but still evokes something. "Chillout' is another dumb term, but I guess it fits here. A slightly more involved example of this is Cascade by Future Sound of London.
I like both different groups in different ways, and for different things. Rarely will you get a Beat Ambient song that you could sleep peacefully too, but sometimes you can get bored easily if you actually listen to a Pure Ambient song.