The site settings are the settings for your entire site. Here you can change the name of your site and the description.
The 'Global keywords' are the terms that google picks up on, by typing precise terms in here you will move up higher in Googles search results, making your site easier to find, it may also bring in extra traffic to your site, from people who would not normally view it.
These site format seetings are global to your whole website. If you want to change how the date or time is diplayed through out your site then you are able to change them here.