I'm inspired to share my experience with glacial erratics from reading Dan McShane's blogpost A Really Big Erratic White Rock in Jefferson County (1/20/2011) and Dave Tucker's blogpost Appeal to Citizen Scientists: Do you know of any Jackass conglomerate erratics? I could probably safely say that I've known some erratic jackasses during my lifetime but have yet to find any Jackass conglomerate erratics.
For the layman, a glacial erratic is a large rock transported some distance by a glacier from the rock's point of origin to the point where the glacier leaves it. The erratic is usually composed of a different type of rock than the rock or geologic conditions at its resting place. It often looks out of character with it's surroundings. Identifying the erratic's rock type provides clues to its point of origin and the sense of the glacier's movement (i.e. direction, distance).
My first sighting of glacial erratics was during numerous hikes through the Sierra Nevada in my formative years when I lived in southern California. The Sierra provide many opportunities to view alpine glacial landforms. The photo below shows a glacial erratic near Carson Pass.
In spite of the dense tree cover and dense development, glacial landforms are much in evidence in the Puget Sound region, and this includes large glacial erratics. I live within spitting distance of the Puget Sound shoreline along Browns Bay north of Edmonds. My favorite erratic is found along the shoreline here. Call it the "home erratic" if you will. At the highest tide it is almost covered providing a safe perch for herons and gulls. At the lowest of the spring tides the entire rock is exposed. I'm told by a diving friend that similar large rocks exist just offshore here. I'm inspired now this coming spring to chip off a piece to see what type of rock it is.