Every website you go to sends HTTP headers to and from the browser. One of these headers is a server header and looks like this:

Server: Apache/1.3.26 (Trustix Secure Linux/Linux) mod_perl/1.26
This header is easily reduced or disabled in Apache, just look at the ServerTokens command in the config file.

http://httpd.apache.org/docs/mod/core.html#servertokens