The CW, owned jointly by CBS and Time Warner (from whence the initials), is the result of the 2006 merger of The WB and UPN. This is a network that is utterly about demographics. Specifically, that demographic which spends more time online that it does watching the tube. A pretty smooth move, since "reaching" young adults means giving them something to talk about on the Internet. Nearly all of The WB's and UPN's best-known programs -- Gilmore Girls, Smallville, Supernatural, America's Next Top Model, Everybody Hates Chris, One Tree Hill, Veronica Mars -- were carried over from those networks.
The network struggled during its first few years, to the point that the Tribune Company, owner of key affiliates like New York's WPIX and Los Angeles' KTLA, dropped CW branding on its CW stations; for example, WPIX (formerly CW11) has reverted to being PIX 11 (the name it carried back when it was an independent station), while Denver's KWGN (formerly CW2) developed a Dork Age of calling itself "The Deuce" with a younger image for a couple years, including removing their older news personalities and attempted to make themselves hip (which made it worse; it's now branded as the comparably staid "Colorado's Own Channel 2"). Recently, though, it's finally found its footing, with the original hits Gossip Girl, 90210 (a Sequel Series to the '90s FOX show), The Vampire Diaries and Nikita, in addition to a number of still-popular shows from the WB days (Supernatural, Top Model). While it's still in fifth place behind the "Big Four" networks (though it's more in sixth place as the Spanish language Univision network has begun to expand), one must remember that The WB and UPN were also like this even at their respective high points, and the CW seems to be more content aiming for the teen/young adult niche (which they do spectacularly well in) than going for broader appeal like the major networks.
On a less pleasant note, the launch of this network was the death knell for African-American-cast sitcoms on network television for the foreseeable future, as it removed UPN, the only broadcast network that was still committed to running those types of programming. When UPN merged with the WB, the latter network's sensibilities wound up dominating, causing black-focused shows like Everybody Hates Chris and The Game to get lost in their new network home's identity. While The Game was lucky enough to make a Channel Hop to BET (and became that network's biggest show ever in the process), other shows of its ilk saw themselves getting bumped off.