Vexillology is the study of flags. The word comes from the Latin "vexillum", meaning flag, with the Greek suffix "-logy" for the study of something. The field covers more ground than the name suggests, running from academic analysis of historic banners to the practical business of designing new ones.
Vexillologists are not simply flag enthusiasts. The work involves the physical object, but also the social role a flag plays and the identity it is claiming, which means most of the interesting questions are historical rather than visual.
Why flags are worth studying
Flags do real work. They carry pride, stand in for an entire country at international events, and sometimes provoke serious arguments about who a place belongs to. Tracing how a country's flag changed will often tell you about its path through colonialism, revolution or reform faster than reading a summary of the same period.
The same applies well below national level. Cities, organizations and political movements all use flags, and the symbolism is usually more legible in those cases because the designs are newer and the intent is documented.
Getting into it
Start with reading. Books and articles on flag design and symbolism will get you the vocabulary quickly, and the North American Vexillological Association publishes a good deal of material aimed at beginners.
After that, find other people doing it. Forums, conferences and community discussions are where most of the working knowledge sits, and experienced vexillologists tend to be generous with it.
Then look at flags in the field. Government buildings, historical sites and international gatherings all show flags in use rather than in isolation, and how a flag is flown often says as much as what is on it.
Eventually you can add to the field yourself, by writing, by designing, or by presenting research.
The digital shift
Online archives, digital collections and interactive maps have made flags from anywhere in the world easy to study without leaving a desk, which was not true a generation ago. Social platforms have done the rest, letting people share findings and arguments with a global audience more or less immediately.
Flags are one of the few artifacts that almost every human society has produced independently, which is what makes them such a useful way in to a place you know nothing about.