Ambient lighting is the general illumination of a room. It differs from task lighting in that its purpose is to provide an even wash of light throughout a room as opposed to concentrated light for a specific purpose. Lamps, ceiling lights, wall lights and recessed lights can be used for this effect.
Task lighting is exactly what it says on the tin. It's there purely to serve a purpose, ie to increase the illuminance of a desk or reading area. However just because it has a job it doesn't mean it can't look attractive. Angled lamps, track lights and spotlights work well for this.
Accent lighting is an effective and simple way to highlight a room's specific features, artwork, paintings etc. Adjustable recessed lights, track lights and spotlights work best for this.
The term decorative lighting covers all the traditional kinds of lighting, floor lamps, table lamps, chandeliers etc. The actual light fitting is there to be seen so the appearance is important.
If you would like some ideas on the best lighting for making your small space look larger, please see our previous Circuit Shorts here.