There are 3 Type of extensions.
1. gTLD ( e.g. .com, ,net, .info, .guru, .bike, .berlin etc ) . There will be approx. 500+ gTLD extension will be available by end of 2014. Already 100+ options available. Most popular is .COM, sharing approx. 35% of Total domains registered in all type of extensions. .GURU / .BERLIN launched recently, and gaining fast ( approx. 40000 registrations in first month ). gTLD's are open to register for all.
2. ccTLD's ( e.g. .US, .UK, .DE, .IN etc ) ccTLD are 2 letter code assigned to every country of world. Any 2 letter extension means ccTLD. Few people confuse .CO like domains with gTLD's, but its a ccTLD for Colombia. a ccTLD is good for business on country level. ccTLD's have different rules for registrations in every country. In many countries ccTLD's allowed to register only from local people, but few ccTLD's are open to world ( e.g. .IN, .TK etc )
3. sTLD's ( e.g. .ASIA, .CAT, .MUSEUM ) These are Community sponsored TLD's i.e. assigned for specific community / business. e.g. .CAT is only for Catalan-speaking community, .ASIA Requires one contact in the Asia/Australia/Pacific region.