If you are in the EU, most graduate schools typically prefer you to have a Masters degree before applying for a PhD, it is apparently not so necessary in the US. The Masters is however, important for several reasons:
- You can see if chemical research is what you really want to do, as most undergraduate programmes have no or very limited opportunity for original research, largely being focussed on equipping you with a set of techniques and skills to enable you to carry out chemical research. I have many friends who were 'all chemistry'd out' after their Masters, and left the field completely (Of my cohort over half are no longer in a chemical field).
- You can try out a field of chemistry in a research environment, either one you have come across during your undergraduate e.g. spectroscopy of small molecules, and enjoyed, or one that only really gets introduced at undergraduate level, if at all e.g. supramolecular chemistry.
- You can try out a supervisor you are considering working under for a PhD, without the commitment of 3+ years on the line.
- It demonstrates to a potential supervisor that you are capable of conducting original research to a good standard. Agreeing to take someone on with no prior demonstration of research aptitude is not something many supervisors would do!
PhD programmes can also vary a lot: Some are longer and have a first 'introduction' year, where they train you in the specific skills you need to complete the programme. This first, often taught year is very similar to the 4th year of an integrated Bachelor + Masters course. Some are just straight research from the start, with an evaluation at the end of the first year, the failure of which prevents you from continuing, and you leave with an M.res or similar. I could go on...
To more directly answer your question, the best way to find out about the differences in day-to-day activity between the different areas of chemistry at PhD level is to try and talk to some current PhD students.
If you have completely free reign and no particular interest, I would suggest you aim to talk to students in most of these categories:
- Physical lots of maths, often boils down to applied physics, in my opinion the least 'chemical' of these options. Can also overlap with analytical in areas like atmospheric chemistry or kinetics.
- Analytical (physical chemistry typically focussing on spectroscopic exploration)
- often overlaps with computational (modelling the behaviour of chemicals on surfaces e.g. catalysts, or the behaviour of proteins and their binding sites, or small molecules as entirely simulated objects, to name a few)
- Chemical informatics e.g. developing tools to help statistically analyse chemical behaviours for drug discovery, or to communicate chemical information electronically. Can be a good way into programming with a chemical slant.
- Synthetic chemistry this comes in many flavours, organic and inorganic, but most are similar on a day to day basis (see below for exceptions)
- Medicinal/natural product synthesis synthetic chemistry but typically much longer hours, often the most internally competitive/'macho' culture in the department
- Materials chemistry this can cover a very wide range of specialisations and include engineering and physics too; very dependent on the particular programme. Nanoscience is sometimes lumped in here, sometimes in organic and sometimes in inorganic!
In broad strokes, more physical PhDs can require greater discipline, because you don't have the obvious 'put this reaction on/analyse this intermediate' type workflow and can spend weeks waiting for computations to run/repairs to a piece of analytical equipment etc. It is easier to remain emotionally uninvested in the outcome of an experiment, because you are typically trying to find an answer to a question, rather than produce a specific result. By contrast, synthetic PhDs can have moments of soul-crushing defeat when reactions repeatedly fail, frequently requiring a lot of mental fortitude to keep trying things to get a reaction to work/work out a way round it.
To summarise, doing a Masters first is a good idea, to ensure you want to go the PhD route and improve the strength of a PhD application. For picking a PhD (and to a lesser extent, a Masters) you really need to be able to narrow down your options by some/all of topic, supervisor, location and type of programme.
addition from comments: Here is a link to information provided by ACS for students looking to find out about graduate courses, should be a good starting point.