To dominate is to take control, as Dallas has said. Domination is an act stemming from discipline, and expecting the same. To domineer is to bully, though it can be used to describe bully-play, force-play and other psychological aspects of BDSM. Domineering is an act stemming from insecurity, and expects the same.
A dominant will take control as per the limits you arrange; usually this is beforehand, sometimes on the fly if they and you trust in one another enough. They will use a mixture of unspoken mechanical handling and verbal command to order you about, move you, position you, and make use of you, and in theory will do so within the aforementioned guidelines.
A domineering person will act arrogantly and aggressively, and if it's not a part of the psych play mentioned previously, will be doing so simply out to get an egotistic sensation from harassing another. In said situations it constantly runs the risk of running past unmentioned limits, simply because it doesn't care to know them, let alone abide them.
So, the final comparison; A dominant takes control out of respect. A domineering person takes control out of a lack of respect.