If you were into computing at all during the 90s, you probably remember getting those disk sets or CDROMS that would be packed full of shareware titles or random small games that, while not big budget, were often something that could keep you entertained hours on end. Millions of puzzle games, board games, et cetera. Today, there's a bit of a vacuum in that market, with almost all game developers focusing exclusively on large-scale projects for action/RPG/adventure/etc. That's expected to change soon, with an estimated nearly $1 billion increase in sales for "casual games" over the next few years. For a lot of people, that's something they can look forward to.
Several trends are helping push casual games out of the margins and into the forefront. Advertising has emerged as a key revenue opportunity for games, and hard-core games reach only one of many demographics that advertisers covet. Also, casual games tend to be small and have minimal processing needs, making them ideal for mobile devices, particularly cell phones.
While "casual games" don't usually have quite the quality or depth of more grandiose projects, many people aren't looking for that. They are looking for something to relax and be casual with, which is probably mind-numbing games like Solitaire are so popular. EA is one of the biggest providers of these "casual games" currently, and others are looking to start.