Cover does factor in since it hints at what the book is about.
Sometimes the cover attracts me, sometimes the title, and sometimes the author.
Other times I simply look through a genre I like, find a book the size I want to read, and read the back.
If there is a positive review by an author I like I'm much more likely to read the book.
If there isn't much of a summery on the back or inside flaps I read a few pages.

Some books I just look at genre though. Especially if I've never read that genre.
Then I'll pick out a book the size I want. I never do this when buying, but at the library I do.
It introduced me to the genre and I can later look at reviews online. If I don't like the book and the reviews say it's the best book they've ever read in that genre, I'm less likely to read it again.
That's kind of the route I took when I read the Twilight series.
I wanted a fluffy teen romance and found that the series was popular so I requested it at the library.
Didn't enjoy it beyond a fluffy mindless read but now I know what to expect from teen romance to a certain extent.

I really do prefer to research the books I want a bit first though.
I hate wasting my time with a book. But I hate not giving a book a chance.