loans are a bitch... the rates have been going up since I graduated. Your financial aid coordinator can tell you exactly what your rate will be, and the rate changes every June. Deferment means you don't pay interest on the subsidized loans, but you DO pay on the unsubsidized loans. Forebearance means you pay interest on everything. Deferment for economic hardship (what you'll be applying under during residency) is a 1-year term, and you can renew it for a total of 3 years. After that, forebearance is probably your only option.
I have read in other forums that these options are changing and deferment may disappear or be harder to get. Right now, if you are a resident with no other income and have more than a minimum of loan debt, you should easily qualify. If you start moonlighting, however... your story may change. To my knowledge, this type of loan interest is not tax deductible, but I've heard of people rolling their loans into their mortgage, in which case it would be. Seems like a tenuous proposition unless you knew you had other income...
Hope that helps.