I wanna to know in detail what is the difference between symbian OS and j2me.Which is best to use
It al depends on what you are trying to build. If you need to support multiple phones with different OS, then JavaME is probably what you want.
If you need to develop full native applications for Nokia phones with Symbian OS, then Symbian is probably what you want.
Although JavaME is not platform independent (you will probably have to have multiple versions your APP for phones sharing the same features) you will get some degree of independence that will make your life easier if you are targeting multiple platforms.