Length overall (aka LOA, o/a, or sometimes oa) is a common measurement of a boat's length. It is also the most intuitive measurement, as it measures the length from tip to tail. It is measured parallel to the waterline, and thus can be properly measured only once the boat is in the water, but other than that is fairly straightforward.
I say "fairly", because there is some debate as to what should be counted. Some people will use only the hull length, while others will include projections such as bowsprits. ISO 8666 standards refer to length of hull (LOH), but those wishing to clearly specify berthing requirements (or to make their ship sound more impressive) may refer to sparred length, meaning the hull length plus bow and stern sprits. Alternatively, you may find references to Thames tonnage, a UK measurement system that specifies LOA to be stem head to the rudder head, which may in some cases refer to a length shorter than the Length on Deck (LOD), as some decks may project beyond the rudder.
When specified properly, this is a useful measurement for workaday, practical purposes, and is a common way to refer to boats (e.g., a 14-foot yacht). It is commonly used to price marina berths, which are often charged by the meter, and is often a factor in determining construction costs. From a scientific point of view, it is a fairly worthless measurement, and the loaded waterline length (LWL) is much more interesting when calculating a ship's performance.