This LD site seems pretty comprehensive place to start:
http://www.ldonline.org/That probably answers part of the "Who's affected?" question.
There are lots of other sites if you google and there are other sites.
I personally dislike the term learning disability.
Many friends I had certainly had learning differences and/or differences in ability/different kinds of minds. They certainly weren't daft and were very competent people, but sometimes the crowded classroom, method of teaching and condescending attitudes of some support staff/other kids impaired their learning.
Change the learning environment/style of teaching and they probably would have excelled.
Some people may find difficulty in spelling, say but find it easier to do creative and visual based work.
Some argue that the rigid "one size fits all" school system is partly to blame for students having LDs and having issues understanding and completing schoolwork. I agree with this view.
I don't personally believe that everyone learns in exactly the same way. Perhaps a high aptitude level in one skill area leads to a low aptitude in another.
I personally think that it's very unfair to pressure someone to be good at absolutely everything. Everyone has weaknesses and no one is perfect.
For instance I learn best when working alone somewhere quiet: I do brilliantly. But, put me in a crowded classroom with lots of people talking all at once, and I can't focus. I am not as good at group-work as I can't initiate or keep track of what people are saying that well.
All the best.
I hope this helps
Thanks for your opnion and the link! This will be very helpful.