A River is a stream of fresh water leading to an Ocean, Lake or Sea. As you can see in the picture, the Colombia River runs through the entire state of Oregon and out into the Pacific Ocean. There are many rivers like this in the world. Rivers can be windy and straight in some parts.
A lake is a large body of fresh or salt water that is surrounded on all sides by land. Lakes are usually inland and can be man-made or natural. Rivers, streams, and creeks sometimes lead into lakes. In the picture here Crater Lake shows that lakes can be very large. Crater Lake is a fresh water lake, surrounded on all sides.
A gulf if a body of salt water that is connected to an ocean, sea or lake. It is also known as a bay. It's partially surrounded by land, like shown in the picture, they are surrounded on three sides. This picture shows the Gulf of Mexico.
Sea's are like oceans because they are also salt water. Sea's are also very much like lakes because they are either surrounded by land or have a small outlet that connects itself to an ocean. A few Sea's in the world are: Carribean Sea, Mediterranean Sea (shown here), Black Sea, Arabian Sea, Red Sea, and the Caspian Sea.
Oceans are extremely large bodies of salt water that extend as far as the eye can see. They are the largest bodies of water on the planet. There are 4 oceans in the world: Pacific Ocean, Atlantic Ocean, Indian Ocean, and the Arctic Ocean. There are also two other oceans that are less known being the Arctic Ocean and the Southern Ocean. As you can see from the picture, oceans take up over half of the earth.
For more information about Bodies of Water, like the ones above, go to:
Social Studies for Kids/Bodies of Water http://www.socialstudiesforkids.com/articles/geography/bodiesofwater.htm
