Anesthesia:
Mostly, you monitor dogs and cats under anesthesia. You record heart rate, respiratory rate, blood pressure, and various other things every five minutes. Every now and again, the animal will begin to move/wake up, so you panic, push the extra propofol, and frantically page the technician. Sometimes your patient's blood pressure does scary things. Usually the surgeons take forever.
You'll spend a day in large animal, where you'll realize how amazing it is that any horse survives anesthesia ever. There's often food in the break room. The techs will probably yell at you when you do something silly. You'll usually get to eat lunch. The attire is awesome (scrubs all day every day). You'll get to place a lot of catheters and do your own intubations. You will almost certainly get called in every time you're on call.
Cardiology:
The hours are fantastic. You start at 8 or 9, and rarely do you finish later than 5. A cardiology inpatient is like a unicorn - a mysterious creature that is rarely sighted. There are lots of adorable puppies and older dogs. There will be the occasional angry cat. You will get exponentially better at hearing murmurs. The rounds are highly useful. You will probably not get called in when you're on call.
Oncology:
The rounds are fantastic. There may or may not be inpatients, depending on...who knows what. I did this rotation twice - once we had no inpatients, once we got many, many transfers. If you don't believe in oncology/have a hard time with cancer, this will be a challenging rotation. Paradoxically, you'll see lots of happy, tail-wagging dogs as well as some really sick ones. The afternoon rounds are when you get pimped - don't try to guess the answer if you don't know.
Ophthalmology:
O-PHTH-almology. This matters to ophthalmologists. You'll see a zillion cases per day and get to see some interesting/horrifying surgeries (phacoemulsification can I get a what what). For some inpatients, your entire day will revolve around eye drops (every 5 minutes for some stretches of time). You will learn which end of the slit lamp to hold. You'll be able to use a tonopen without hurting yourself or others. I don't think we rounded more than once. I was oddly okay with that.
Orthopedic Surgery:
If you like elective surgeries, awesome. Otherwise, this rotation might suck for you. The hours are terrible. The chiefs do not care about this. There will be more surgeries scheduled than can physically be cut during the week. The dogs are all large and most of them are badly behaved. The techs are no-nonsense and fantastic. The resident probably hasn't slept in the last three days.
Radiology:
The hours are 9 to 5. The rounds are useful. The chief-resident rounds in the morning are where the residents get torn new orifices while the students watch in horror. You'll learn how to take good radiographs and which end of the ultrasound probe to hold...if you want. Or you can sit in the student rounds room and eat doughnuts. It's really up to you.
Theriogenology:
You will spend countless hours with your arm up a horse's rectum (+/- the ultrasound probe). You will rapidly acclimate to the horrid smell of plube (poop + lube. Kind of like Santorum, except this variety is not the by-product of anal sex). It's very exciting when you realize that you can semi-competently ultrasound a mare's reproductive tract.
You will become intimately familiar with semen. You may have several terrifying experiences trying to capture an erect, thrusting stallion penis within an artifical vagina. All sex jokes, all the time. There will be foals. They are adorable and mostly naughty. The hours are terrible. You will probably hate getting up at 2 am for mare checks (ultrasounds every 6 hours to monitor for ovulation).