Can any one tell me some important functions in wordpress.
Well it depends on what exactly you wish to do in wordpress. Best place to look for the basics is here - http://codex.wordpress.org
I can certainly understand if you feel a bit intimidated. Last time I counted was a few versions ago (3.0.1)
Total number of PHP files searched: 269
Total number of PHP files with Classes or Functions found: 167
Total number of PHP classes found: 168
Total number of PHP functions found: 3856 - [ver. 3.0.1 - 3859 functions]
That's a lot and that doesn't include any found in plugin or theme files!
Though idle perusing might spark some ideas, as jaagare said, better to get to know them on a "need to know" basis.