The Gulf Stream is a current that runs northeastward along the Atlantic coast of North America. Because it flows north, it brings warm, tropical water from the Gulf of Mexico to fairly far north along the east coast, resulting in eastern states as far north as Massachusetts having a surprisingly tropical climate, at least during the summer.

The cause of the Gulf Stream is the rotation of the Earth: the Earth rotates faster at the equator than it does farther north (because of the geometry of a sphere - think about a spinning top, where the fastest-moving part of it is also the widest part). As a consequence, the Earth is spinning eastward underneath the tropical oceans, resulting in the oceans having currents flowing toward the west (this happens in winds, too, but when we normally talk about wind direction, it's wind OUT OF a direction, so the tropical trade winds blow out of the east, even though they're going west). However, because the Earth is spinning less fast at the poles, the flow is reversed: polar currents flow toward the east instead of the west. In order to keep water from piling up, this means that the currents on the east coasts of continents flow away from the equator, and currents on the west coasts of continents flow towards the equator. This is also why the current in California is cold, by the way: it's flowing out of the Gulf of Alaska.