As a noun:
* A taxi: "I took a cab to the airport."
* A compartment in a train: "We booked a first-class cab on the train."
* A short form of "cabin": "We stayed in a log cab in the woods."
As a verb:
* To ride in a taxi: "I cabbed it to the restaurant."
* To transport in a taxi: "We cabbed the guests to the hotel."
The specific meaning of "cab" depends on the context of the sentence.