TLD, sometimes called a domain extension, is the last part of a domain name (not the whole address). For illustration, the TLD for www.junglenavigator.com/faq/ is ".com".
It is a TLD with at least 3 characters (e.g. .com) that operates directly under the policies established by ICANN processes for the global Internet community. Some can be used freely, some are reserved for special usage (e.g. .gov) and are called sponsored top-level domains (sTLD). Simply put, a generic TLD is any other than a country top-level domain (ccTLD).
It is a TLD used for a country a sovereign state, or a dependent territory, e.g. .at for Austria.
Yes, those that are marked by an arrow are already registrable.
This site gathers information (mostly) on new gTLDs and presents it in a convenient way so you won't miss out on the latest news from the great gTLD scramble. Watch this site and become aware of every change and chance arising around new gTLDs.