A lake is a naturally occurring body of water that is completely surrounded by land, whereas a sea is an arm of the ocean that is only partially surrounded by land masses and is composed of salt water. Not all bodies of water are named appropriately; for example, the Caspian Sea is actually a lake since it is not connected to the ocean.
A lake is a large water body that is landlocked while a sea is a part of the ocean that is covered by land on most areas. Apart from being deeper than lakes, seas are always bigger. Lakes can be salty or fresh in nature, but seas are always salty.