Chromecast only support HDMI?

yes

I know chromecast kinda look like a USB stick...

can someone explain to me how this work?

Read this Flora: http://www.tomsguide.com/us/what ... ast,news-17637.html

too complicate....still like to old way to watch my RMVB on TV

it support all video format?

Most of them....but no RMVB.  Video - AVI (Xvid, AVC, MPEG1/2/4), MPG/MPEG, VOB, MKV (h.264, x.264, AVC, MPEG1/2/4, VC-1), TS/TP/M2T (MPEG1/2/4, AVC, VC-1), MP4/MOV (MPEG4, h.264), M2TS, WMV9, FLV (h.264) Photo - JPEG, GIF, TIF/TIFF, BMP, PNG Audio - MP3, WAV/PCM/LPCM, WMA, AAC, FLAC, MKA, AIF/AIFF, OGG, Dolby Digital, DTS Playlist - PLS, M3U, WPL Subtitle - SRT, ASS, SSA, SUB, SMI

but I just couldn't understand why do we need chromecast or apple tv or roku....if just need to steam video and watch on TV....

flora, chromecast is mainly for online streaming (youtube, netflix, viki.. etc).  but can also stream any website from desktop chrome.  So, if I go to a site with tbb drama, I can play it on chrome, and tell it to stream to any of my tv that has a chromecast plugged in.  In order to stream local files, you can either directly open the video file in chrome and stream, or use Videostream extension.

flora, yeah there are many options.  I got chromecast on all my tv and projector mainly because chromecast is the cheapest option.    Here, it was only $30.  And a lot of time they have promotion, which comes with $20 Google Play credit.  I just sold the $20 credit for $17, so my chromecast was $13 each.  Good deal lor.