please tell me the difference between that, i know it use to serve same purpose.
Not necessarily the same purpose.
In a nutshell,
Studio mixers are used to sum together the final soundtrack of a film or specific sections of it (foley, gunshots, explosions, sweeteners, loop group, etc.). They could also be used for recording ADR. They could also be used to create sound effects.
Field mixers are used generally to mix down two or more mics being used on set to a useful track that the film editors can use in their editing process - i.e. the film editors will not load all iso tracks recorded on the day, but a simple stereo mixdown of the mics via this mixer. You could also use them to solely record the iso mics via custom EQs or filters or level adjustments.
This is in my opinion, the most common uses of these pieces of equipment. I know for a fact that it all depends on workflow and what your project calls for that determines how you are going to use these. There are infinite variations and ways of using them it would take more than it's worth to try to describe them all.