I'm not going to get into to much detail, but lighttpd uses far less memory which results in a smaller server load. Here is an interesting website that explains more of the pro's and cons. http://www.phpmagazine.net/2007/02/lighttpd_vs_apache.html
I agree with HostPenguin. Litespeed consumes less memory then Apache, but I like working with Apache since I am more familiar to Apache then Litespeed.
I love apache because of the mod_rewrite mod. Most SEO/SEF components use Apache's mod_rewrite rule. If you want to use Litespeed you will have to write the rules urself.
Lighttpd is not Litespeed. These are entirely different pieces of software written by different people. http://litespeedtech.com/ http://en.wikipedia.org/wiki/LiteSpeed_Web_Server Litespeed is entirely compatible with Apache config files. Litespeed has a variety of prices ranging from $150 per year or $249 once only to $999 per year or $1999 once. Pricing is based on the number of CPU cores. Litespeed has a license file that is required to make it work. The license file is tied to the computer in question and may break if some parts of the computer are replaced. Litespeed has commonly used Apache modules such as mod_rewrite built in. Litespeed has a 500 concurrent connections limit on a VPS with the "Enterprise" edition and has 150 concurrent connections on a VPS along with many other limitations with the "Standard" edition. Litespeed charges money for the 15 day trial of the "Enterprise" edition. Litespeed's EULA is somewhat restrictive. It is non-transferable which means you can't sell your license to anybody else. It may also mean that you can't have it transferred to another computer. You are also not allowed to use it for "any illegal activity or to host pornographic content." Which is a very broad, ambiguous statement that could be interpreted many different ways. Litespeed doesn't allow you to hide the fact that you are using Litespeed in the response headers. http://www.lighttpd.net/ http://en.wikipedia.org/wiki/Lighttpd Lighttpd has its own config file format and is not compatible with Apache config files. Lighttpd has one price: free. Lighttpd has no license file and will not break if you move it to another computer or upgrade parts of the computer. Lighttpd has its own version of commonly used Apache modules such as mod_rewrite but with their own configuration language. Lighttpd has no artificial limitations. Both of these web servers are light weight however only Lighttpd appears on the comparison of light weight web servers page on Wikipedia. Both appear on the comparison of web server software page on Wikipedia. http://en.wikipedia.org/wiki/Comparison_of_web_server_software http://en.wikipedia.org/wiki/Comparison_of_lightweight_web_servers
Litespeed is what I just moved to, and its been holding up whereas my Apache server would crash every 7 days. I've rebuilt it several times, even to defaults, and it still crashed. So far litespeed has been performing well, although it isn't under fulll load just yet because I haven't moved everyone over, but it's working well and if it continues to work well I'll keep it running. Their TOS is a dumb one though.
Apache 2.2 is very stable and performance is very comparable to lighttpd. I would stick to Apache if only because it has broader support.
litespeed can help stop fake apache requests or spam, and it speeds the process of loading pages up and uses less cpu. But it is very expensive for a web server.
I think if you got money and if you are looking for performance and scalability, then you should go for Litespeed. The main performance of Litespeed is on Static pages. It loads the static pages 6 times faster than the time which apache takes to load. Also the scalability is also excellent for Litespeed when compared to that of apache. As Litespeed is not free, it has its own support which can be contacted if you have any issues. Also PHP and Ruby on Rails comes as integrated with Litespeed.
The only thing I found missing from litespeed so far and they are working on it, is flength. So if you have like PHPMotion or similar script, I would stay away from litespeed until a stable version of 4 comes out with the SSI and the loading bar, unless you can get help to find a solution to that.