A map scale is the size of an object compared to the size of the object's smaller representative on a map. This can be shown by a scale bar and a ratio 1:n. This lets the person using the map measure a distance on the map and know what is the distance on the ground.
People sometimes speak of a map as one with a "large scale" or "small scale". In this context scale is obviously relative. It is safe to say that a "large scale" map displays objects such that they appear relatively large. For example, an island displayed on a 1:10,000 map will appear larger than if it were displayed on a 1:25,000 map. Thus, the former is "large scale". What can be confusing is that for a map of a given physical size, say 11 by 17 inches, a "large scale" map will have a smaller geographic extent than a "small scale" map centred on the same point. Maps with a ratio of 1:50 000 or larger (for example, 1:40 000 would be larger) are considered large scale. Maps with a ratio of 1:50 000 to 1:250 000 are considered intermediate scale. Any maps with a smaller scale (for example 1:300 000) are considered small scale. (indirect quote from http://geography.about.com/cs/maps/a/mapscale.htm article titled: Map scale By: Matt Rosenberg)