Academic probation is essentially a 'final warning.' You're admitted into a general program in whatever faculty and take courses as if you majored in the subject (ie. biochem courses, even if you had an unacceptable average for a biochem program).
If your grades improve you reapply for admission into the program...if your grades don't improve, you have to withdraw from the university.
You don't want to be on academic probation, simply because you'll have it hanging over your head all year...the pressure's on, in some sense.
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To Monica: Your friend should re-evaluate their study habits. It seems as though, given history is what she'd like to major in, the interest is clearly there...so somehow when being put into practice, her studying just didn't pay off (so the way she's studying has to change), or she somehow felt prepared without putting in a proper amount of time (they were her first set of exams, it's not uncommon to be 'caught off guard'). Perhaps she has difficulty writing papers, etc.
Have her reflect on what she's done, and go and visit the Centre for Student Development. They have various resources at your disposal that I'm sure could help her out.
EDIT: Have a look at this webpage to see what the CSD has to offer:
http://csd.mcmaster.ca/