In some situations, you may want to only allow people with specific IP addresses to access your site or you may want to ban certian IP addresses this will only work if you know the IP addresses you want to ban and, as most people on the internet now have a dynamic IP address, so this is not always the best way to limit usage.
You can block an IP address by using:
deny from 000.000.000.000
You can allow an IP address by using:
allow from 000.000.000.000
If you want to deny everyone from accessing a directory, you can use:
deny from all
but this will still allow scripts to use the files in the directory