Roots rock is a genre of music that finds its basis in blues, country, reggae, and folk. As its name suggests, artists in this field are modern artists who attune themselves to the "roots" of modern music. Often times this merely means they are a direct throwback to older times - witness Merle Haggard, whose recent recordings are directly derivative of (surprise!) his older recordings. Thus, Merle Haggard is somewhat of a meta-reference, since he is both the "roots" and the modern portrayer of the roots.
Although you could argue pretty much any modern artist can be traced back to the blues and early American soul and folk music, roots rock is specifically geared in artists who tend towards simpler arrangements (often merely a voice and an acoustic guitar) and simple lyrics to express their emotions and opinions.
Modern roots rock artists would include the likes of jam bands like Phish, Guster, and Rusted Root; anti-folk and modern folk artists such as Adam Green, Shawn Colvin, and William Lee Ellis; alternative country icons such as Whiskeytown, The Handsome Family, and Son Volt; and the handy "world music"-themed acts such as Tejano stars Los Lobos, reggae crown prince Ziggy Marley, and the Irish stomp of Black 47.
A lot of times roots rock is pre-empted today by the wink wink atmosphere of postmodernism, or even worse, the bland sincerity that often accompanies the plagiarizing of artists who have come before you. As Picasso famously said, "Great artists steal"; it takes a true talent to bring the music full circle, back to its roots but connected to the present in a meaningful way.