Beach glass and sea glass are different names for the same thing. People near oceans usually refer to the glass found on the beach as sea glass, while those of us near lakes call it beach glass.
Beach glass or sea glass is glass that has been discarded (or purposefully dumped!) into an ocean or lake. It may have been lost in a shipwreck, or during a natural disaster, or it may have once been a bottle left on a beach after a campfire. The mysterious origins of the glass add to its unique qualities.
Over the course of many years, the action of the waves and the sand smooth the surface of the glass, much like sandpaper smooths the rough surfaces of wood. Sea pottery is created in the same way as beach/sea glass.
Some sea glass has a frosted surface. This is called hydration, and is caused by the lime and soda (components added during glass making process) being leached / pulled out by the constant contact with the water.
When found on the shore of an ocean or sea, the glass is usually referred to as sea glass. When found on the shores of lakes (including the Great Lakes in the United States), the glass is locally referred to as beach glass.