A subwoofer is a large speaker (usually 8 to 15 inches across) that plays the low notes of the music. Being so large, they need a lot of power to create these frequencies. When you hear a car that "thumps" down the street, it has one or more powerful subwoofers to do that.
Bass frequencies commonly get as low as 30hz or less (30 sound waves per second). Compare that to the high notes in music, which can go higher than 20,000hz (20 thousand sound waves per second).... and you can understand why a large subwoofer isn't able to create the high notes, it's simply too heavy to move that quickly.
A tweeter is small, usually less than 2 inches across. It plays the high frequencies. Being small, they are far more effiecient, and can be loud using less power than a subwoofer. They're unable to move the large amounts of air that a subwoofer moves. These tradeoffs are why the best sounding systems use a variety of speakers, no one speaker can do everything.
There's also mid-range speakers that fill in the frequencies between the tweeters and the subwoofers. These are intermediate in size, usually between 4 and 6.5 inches. There are also many mid-range speakers in oval shapes, to fit the cramped places auto manufacturers put them.
Many times, you'll see 2-way or 3-way speakers.... with a mid-range speaker and a tweeter bundled together to make a single unit. this is done to save the space and extra work or wiring separate speakers, but the effect is the same.... each speaker cone has it's assigned frequencies to handle.