What is the difference between .tk and .com sites

As already said, .tk sites are sites registered to Tokelau, in New Zealand, which gives domains free of charge under some restriction. You must get, for example, a minimun of hits every month and so on. Basically, it's a common "regional" website, while the .com are commercial and international websites. It really depends on what you want to use it for. For a common activity, I'd suggest a .com, since it looks and sounds more "trustworthy" to an average user, while a .tk could seem a bit "suspicious", since often those free of charge sites are used for not so good activities...

tk domain is free and have many limited feature and our site must be have minimum visitor to continue actively,,,i already setup my site using dot tk but now my dot tk domain link is broken and i don't know why because i think my site already have more traffic to reach minimum visitor require

One is a geographical domain the other .com means commercial..so if you want an international site normally.com is better if your site puposes is regional or loceted in the that area better regional domain

its just like you have .com .nl .co.uk etc... .tk is the Internet country code top-level domain for Tokelau, a territory of New Zealand located in the South Pacific.