well in normal waking consciousness we have more control over what we allow into our consciousness (we are selective over what we allow into our consciousness)
I think of it more like this: attention is more just a broad definition, it is a concentration of mental activity that involves focussing on certain stimuli while ignoring others.
selective attention is a type of attention. we are more selective (choosy) about what we focus on.
I doubt you'd get a question in the exam that'd ask you to distinguish between attention and selective attention. there are two types of attention, selective and divided. if you got a question about attention (disregarding selective or divided) it'd probably just ask you to define it.
in short; selective attention: (this is just my interpretation, someone else may know better) we choose (select) to focus our attention (or mental activity if you prefer) on specific stimuli while ignoring others
but as i said, selective attention is a type of attention and you'd probably only be asked to define attention if it came up...

this reply just did a full circle, sorry