I agree with rickh that uname is a place to start, although that gives the kernel version, e.g. on this box it's 2.6.11-x1
As noted, the directory /etc contains files with names that have the string release
in them. You could search for those. Sometimes more than one might exist.
For example, on my Debian-derived Xandros system I have the files and contents noted below ... cheers, makyo
5 DISTRIB_DESCRIPTION="Xandros Desktop OS"
1 Version: Xandros Desktop OS 3.0.3 Business
2 Build Info: 2005-10-31 15:14
3 Part: DBX303ENCD1SE28
4 Sku: DSKB300NAEN
5 xDMS: supported