Wordpress Plugin Development: Do_Action Vs Add_Action

I have started to get into wordpress plugin development. Its just a start and there would be a lot of things that I won't understand and would require some time but there is one thing (in fact two) that I need to discuss specifically, because they appear to be quite important and I don't get them at all.Actually, it is the difference between do_action and add_action and similarly the difference between apply_filters and add_filter. It is quite a basic concept and I think if I go pass it without properly understanding it, I might be in trouble when dealing with complex problems.

Isn't it that you first have to add the filter, then apply it in some place? Like here http://wordpress.stackexchange.com/questions/16573/explanation-for-apply-filters-function-and-its-variables

I guess its not like that. I have been reading a book about wordpress plugin development. And from what I have read till now, I am assuming that add_filters is used to add a filter to a hook during execution of the wordpress code. So it is the practical part which has to be used when working with filters. On the other hand, apply_filters is used to create new filters (but I am not sure about it as I don't understand what exctly it does). It has confused me alot and I don't want to go further ahead without properly understanding this basic idea.