In the spirit of fulfilling the thread title I think I might chime in here and offer a bit of a current perspective on the job as a junior; Mana, Pi & Chinaski can probably add their perspectives too if they feel like it.
At the moment I am in my second postgraduate year of work after studying for 6 years at medical school immediately after graduating from highschool, I am 25. My overall job consists of rotating through different terms on a ~10 week basis (but sometimes fortnightly) & my day-to-day job varies drastically depending on what rotation I am on. So far this year I have been moved from vascular surgery to palliative care to general surgery to cardiology to emergency to rural rotations & later in the year I'll be in the ICU.
Each of these jobs requires something different of you & often they will have very different workloads - vascular surgery was minimum 10 hour constantly busy days where I would often miss lunchbreaks & had frequent overtime spent in theatre when things went wrong at 4pm, palliative care was much more strict with rostered hours & slower paced but I spent a lot more of my day talking with a lot of people which tired me out, cardiology was a heinous mixture of medicine/surgery where patients flew in and out of the ward often without enough time for me to learn their names, emergency is typically a hard slog 10 hour shift without any reprieve due to constant influx & pressure to see people, & my current rural term is essentially me alone overnight in a hospital with a phone to call for help if I need it.
Often I don't know what my job is going to be until I start and then when I do figure it out we change jobs & have to start all over again. Some of the jobs are standard 8pm - 4pm Monday to Friday, some of them are rotating shift work (days one week, evenings the next, nights the week after or some variation) and others are specifically late or night time shifts. You get some say in what rotations you want to do but for the most part it's rare to get everything you want without also being given something you don't want - i.e. seeking out a coveted ICU / Paeds / ObsGyn / Anaesthetics term will probably result in you also getting an entire term of nights or something similar. This is the reality for the majority of junior doctors from internship until they either begin a non-rotational job or start on a training program. I got my ICU term but have been sent to a rural hospital in payment.
In terms of job security it depends what you mean by that - it's unlikely you would ever be OUT of a resident/junior doctor job if you wanted one & were competent but it's also unlikely you would get a job and be able to say "I'll be here for 5+ years". Applications need to be done yearly around July and staying at the same hospital in the same job isn't always guaranteed. Most of my friends who have applied for jobs haven't found out & won't find out where they are next year until November. Rotational jobs (as described above) are generally largely available / easy to retain but don't guarantee you anything specific. Non-rotational jobs or moving up in the job ladder often necessitates moving around / to the job you want and quite frequently this means moving yearly. Even when you're in specialty training the same thing happens - each year of training has specific requirements & you go where those requirements can be met.
Non-rotational jobs as a junior (e.g. a year of paediatrics, medicine, surgery or emergency) are generally considered stepping stones towards a training pathway and as a result they often come with increased expectations - after all, you're likely trying to both become competent enough for the next level job & impress someone so that they will hire you in the future on a training pathway. They give you the opportunity to become a bit more comfortable in your job but this is often coupled with increasing responsibilities as your seniors get more comfortable with your abilities, the end result (anecdotally, I've not been there) is hopefully a constant progression in your knowledge that is borne out of constantly testing yourself. This same idea is carried over into specialty training where you are progressively faced with higher & higher expectations / responsibility for making decisions while focusing on a more specific area of medicine. Thrown into this mix of a job that constantly gets harder as you get more competent is the need to do exams +/- research +/- maintain your sanity.
The end result is that in one way or another medicine often ends up pushing people fairly hard either through high workload, constantly changing jobs & expectations, not being confident on where they are working for the next week or next year, & generally through the actual job itself. I got into medicine not having any idea whether I wanted to do it (I liked chemistry & engineering) and figuring it opened up a lot of potential doors & I'd find something I liked - I'm glad I have because if there wasn't something that I loved about the job then it would be pretty crappy & my engineering friends make way more money than I do.