Why do some urls not have www. in them?

It is because of the way the site manager has set up the forwarding in their DNS records.

Computers are in reality extremely stupd and only understand bits and bytes.. and on the internet everything is based upon the ip address which is currently in the form of xxx.xxx.xxx.xxx (replace xxx with a digit from 0-254).. Humans on the otherhand find ip address's extremely difficult to remember so what was set up was a linked group of computers that were authorities on certain names i.e. we have to take www.yahoo.com apart starting from the right to the left .. your computer will query the nameserver for the .com group first and this computer/network will reply with the single ip address of the authority of the yahoo.com system so yahoo.com has to set up the same type of system to respond to ftp. www. etc the yahoo dns (domain name server) would then give you the ip address of the server that has the ftp/www server and the web server software is programmed to respond to requests on port 80 (standard http) in the format of an "http get" or "http post" request and if you just use the base address (name) of the web server which on yahoo.com without say the "www.yahoo.com/index.htm" or "www.yahoo.com/index.html" it will return the configured default file using http post format.

For example with iis 5 web server software I can configure the default file to respond to any of about 10 file names i.e. index.htm index.html index.asp default.asp default.htm default.html in which the extension of the filename (htm/html/asp) tells my web server software what to expect whithin the file itself..

Actually the filename itself really doesn't mean anything but the internet community has set standards and most people follow them..

don't forget that the name server (which translates "names" into ip address's can be extended so a particular server can be only limited by your imagination.. i.e. www.abc.def.ghi.klm.mno.pqr.xyz.123.com..

in which something has to respond with the ip address and default webpage for this working backwards.. (master domain server -> 123.com name server -> xyz (web server or xyz name server) until the entire name is resolved.

There actually is quite a lot of activity that is going on behind the scenes that you as a human really don't have to concern yourself with.. programmers on the otherhand have to concern themselves with everything but usually just concern themselves with a particular item and let an authority concern itself with the rest of the housekeeping.