A mastering engineer will want a mixed down File wav or aiff usually preferably at 24 bit 48 khz however if you only work at 16 bit 44khz then that is fine basically the mastering engineer will probably tell you what formats he/she accepts and what format mp3 wav etc you want the output dependant on what you want the output for. If you are going to produce 12's from it it is done differently so they would need to know this also.

The mastering engineer may refer the project back to you if there is a major problem in the mixdown that they cant deal with for you to redo.

A mix engineer some one who is going to produce a mixdown of your track would want as much as possible ie the reason file or individually bounced tracks. they would produce the mixdown which would then go to the mastering engineer.

I hope this clarifies what different peoples roles are and what they need to get the job done. Look up an online mastering service and see what they say they accept.

If you want a mix engineer you could pm me.

Thanks guys, that's great.