the base, collector, and emitter
Every BJT has exactly three legs, and knowing which is which is the first thing to learn — wire them wrong and nothing works. Think of a one-lane bridge: cars (the main current) flow from the EMITTER to the COLLECTOR, and a gatekeeper at the BASE decides how many get through. The emitter is the source of the carriers, the collector is where they end up, and the base is the small control terminal in between.
The names describe what each leg does. The EMITTER is heavily doped so it can emit a generous supply of carriers (electrons in an NPN). The BASE is deliberately thin and lightly doped so that, once the base-emitter junction is forward biased (about 0.6 to 0.7 V for silicon), the emitter's carriers flood into the base but mostly do not stop — they get swept across to the COLLECTOR by the reverse-biased base-collector junction. Only a small leftover fraction exits the base as base current. By Kirchhoff's current law the three currents must balance: the emitter current equals the collector current plus the base current, Ie = Ic + Ib. Since Ib is tiny, Ic and Ie are almost equal.
A practical headache: the three legs of a TO-92 plastic transistor are not in a standard order, so you must check the datasheet for the pinout every time — for a 2N3904 viewed from the flat face with legs down it is E-B-C, but a BC547 is C-B-E. Get it backwards and you may forward-bias the wrong junction. The collector and emitter are NOT interchangeable even though both connect to n-type material in an NPN; the doping levels and geometry differ, so a swapped transistor will have a feeble, useless gain.
Measure it: in a working NPN amplifier you should read about 0.65 V from base to emitter and a larger, more variable voltage from collector to emitter. If base-emitter reads 0 V, the transistor is off; if it reads 0.7 V but collector-emitter is near 0.1 V, it is fully on (saturated).
The base-emitter voltage tells you whether the transistor is even turned on.
The arrow in the schematic symbol always sits on the EMITTER and points in the direction of conventional current: outward for an NPN (Not Pointing iN), inward for a PNP. That arrow is how you tell the two apart at a glance.