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A transistor in a circuit has three currents associated with it - base current (Ib), collector current (Ic) and emitter current (Ie). Base current enters the base and exits the emitter. Collector current enters the collector and exits the emitter. Emitter current is the sum of base and collector currents.
Collector current is related to base current by a relatively constant factor called beta or Hfe. A typical small signal transistor data sheet will indicate an Hfe of 70 - 300 meaning that the ratio of collector current to base current of an individual transistor of that type can be anywhere in that range.
Because a small current controls a much larger current, this allows the transistor to be used as an amplifier. If the amount of current supplied to the base is large then the amount of collector current causes the transistor to saturate or to conduct so hard the voltage from collector to emitter is nearly zero. This makes the transistor into a switch.
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