Popular literature is distinguished from artistic literature in that it is designed both to appeal to a large range of people and to entertain. It usually does not contain, nor does it seek to contain, a great deal of the artistic elements of fiction such as complex character development, themes that touch upon the major conflicts of literature, imagery, etc. Popular literature usually does not endure throughout the ages, nor is this the intent of it. This does not mean, however, that artistic literature cannot be popular. It's just that this is not the main intent of artistic literature.
With the invention of modern printing techniques, and most recently the Internet, almost anyone can write almost anything. That also means that almost anyone that can read can find something in the realm of popular literature that appeals to him.
Popular literature can include novels, magazines and nowadays blogs. Within this broad category are many sub-genres. For example, many people enjoy reading mystery stories and there are tons of popular mystery writers. Others enjoy reading so-called "romance" novels. Another popular sub-genre is the horror story.
If you go into any bookstore and browse the shelves, you will see that a great majority of the material fits into the category of popular literature. You have to go to a special section of the bookstore to find the "literature section" or sometimes what is called "the classics". In these sections, you will find artistic literature, the literature which has endured throughout the ages - Shakespeare, Mark Twain, Herman Melville, Nathanial Hawthorne, Jane Austen, etc.
As to theories of popular literature - I'm not sure what this means and perhaps someone more knowledgeable will answer this part. We English teachers tend to be literary snobs, so my definition of a theory of popular literature would be: WRITE WHATEVER SELLS.