Sometimes people complete a Masters degree, but this isn't always necessary, and so we find ourselves as PhD students. Just how you find that PhD course (or program) isn't always easy, but there are a few tried and tested methods. Positions are advertised in the back of Nature, Science, New Scientist and online. Universities let each other know about positions, and finally the networking that you did in your extra-mural year might just come in handy too. Choosing a PhD course wisely is vital. The institute name will follow you around on your CV, so prestige helps. The work environment is important too. You will be spending at least three, and maybe as many as seven years there, so pick one that's not going to have you spending all your time in a damp and draughty basement. Your mentor/advisor/supervisor/principle investigator (PI)/boss is the last and most important step, and one I cannot emphasize enough. This person will likely become the template for you in future life as a scientist, but if they're a crazed megalomaniac the next few years could be hell on earth.
So, you've found a PhD course. It's here that cultural differences begin to become greater, as US graduate students discover another 2+ years of classes and exams, but in the UK, Australia and other parts of the world, day one involves you, a fresh lab book, and, if you're particularly unlucky, being locked in the supply closet. Life is much like it was as an extramural student, with a few differences. When a new undergrad turns up in the summer, you get to boss them around and lock them in the supply closet. There's more personal responsibility, they pay you (slightly) more, and every so often the department head will ask you something in a lab meeting, giving you the chance to shine or embarrass yourself. Otherwise things haven't changed much. Techs still look down on you, postdocs look down on you and the faculty do too.
Now, a strange thing starts to happen. If you've got this far, it's safe to say you're probably feeling a little proud. You're very bright, or at least you think you are, and walk around campus pretending to be Ice Man. Pretty soon however, it begins to sink in that you're not such a big fish after all, and the pond can look more like a lake. Not only are there other people here that are as smart as you, some of them are actually smarter and others that work twice as hard. This shock can take some time to overcome, and often infuses a little humility into ones character. Other times it might contribute to becoming the aforementioned megalomaniac.
I'm not sure what you do all day if your PhD is in one of the humanities, or something like maths, but in life science, it means experiments, but this can mean many things. You could work on a clinical project. This is good if you don't like working with animals, but has a few other drawbacks. Firstly, you're at the mercy of the clinicians who collect the samples. There's sometimes an innate resentment between scientists and medics. They get to call themselves Dr without an advanced degree, and yet they're the ones who always get made department head. You might be strictly cell culture based, which means you get to spend your days sweating in a cell culture room playing with plastic flasks and plates full of red/pink liquid. (Always remember, spray OR flame, never both!) Also, now's the time to switch to nitrile gloves if you might develop a latex allergy. You might even do such cool work that you end up in a BSL (biosafety level) room. I don't know if they let PhD students into BSL4 rooms. I hope not.
You'll probably have to work on animals at some point. Mice and rats are most common. Some labs use bigger animals, but cost and cuteness tends to make that more and more uncommon. Other projects could see you working in the field, chasing antelope or sleeping with meerkats. It probably doesn't bear saying, seeing as you're reading this, but proficiency with technology and computing devices is a serious advantage. The glory that is Pubmed revolutionized life as we knew it when it came to keeping abreast with the literature. If you took a series of time lapse photos of the fellows room (in the US a common room) I spent my time in, you'd see two old and unreliable PCs running Windows 98 and shared by 12 of us become a laptop on almost every desk, but expect to see a lot of two-finger typists.
