Main Difference
The main difference between Length and Furlong is that the Length is a measured dimension of an object and Furlong is a A unit of length equal to 220 yards still used widely in horse racing.

Length
In geometric measurements, length is the most extended dimension of an object. In the International System of Quantities, length is any quantity with dimension distance. In other contexts, length is a measured dimension of an object. Length may be distinguished from height, which is vertical extent, and width or breadth, which are the distance from side to side, measuring across the object at right angles to the length. For example, it is possible to cut a length of wire shorter than the wire’s width. In most systems of measurement, the unit of length is a base unit, from which other units are derived.
Length is a measure of one dimension, whereas area is a measure of two dimensions (length squared) and volume is a measure of three dimensions (length cubed).

Furlong
A furlong is a measure of distance in imperial units and U.S. customary units equal to one eighth of a mile, equivalent to 660 feet, 220 yards, 40 rods, or 10 chains.
Using the international definition of the inch as exactly 25.4 millimetres, one furlong is 201.168 metres. However, the United States does not uniformly use this conversion ratio. Older ratios are in use for surveying purposes in some states, leading to variations in the length of the furlong of two parts per million, or about 0.4 millimetre (1⁄64 inch). This variation is too small to have practical consequences in most applications. Five furlongs are about 1 kilometre (1.00584 km is the exact value, according to the international conversion).

Length (noun)
The distance measured along the longest dimension of an object.

Length (noun)
duration

Length (noun)
The length of a horse, used to indicate the distance between horses at the end of a race.

Length (noun)
Distance between the two ends of a line segment.

Length (noun)
The distance down the pitch that the ball bounces on its way to the batsman.

Length (noun)
Total extent.
“the length of a book”

Length (noun)
Part of something that is long; a physical piece of something.
“a length of rope”

Length (verb)
To lengthen.

Furlong (noun)
A unit of length equal to 220 yards, ⅛ mile, or 201.168 meters, now only used in measuring distances in horse racing.

Length (noun)
the measurement or extent of something from end to end; the greater of two or the greatest of three dimensions of an object
“the fish reaches a length of 10 inches”
“the delta is twenty kilometres in length”

Length (noun)
the quality of being long
“the length of the waiting list”

Length (noun)
the length of a swimming pool as a measure of the distance swum
“fifty lengths of the pool”

Length (noun)
the length of a horse, boat, etc., as a measure of the lead in a race
“the mare won the race by seven lengths”

Length (noun)
the extent of a garment in a vertical direction when worn
“the length of her skirt”

Length (noun)
the full distance that a thing extends for
“the muscles running the length of my spine”

Length (noun)
the full extent of one’s body
“he awkwardly lowered his length into the small car”

Length (noun)
the metrical quantity or duration of a vowel or syllable
“the length of the syllable is isomorphic with the length of the syllabic vowel”

Length (noun)
the amount of time occupied by something
“delivery must be within a reasonable length of time”

Length (noun)
a piece or stretch of something
“the surviving length of track”
“a length of brown satin”

Length (noun)
an extreme to which a course of action is taken
“they go to great lengths to avoid the press”

Length (noun)
the distance from the batsman at which a wellbowled ball pitches
“Lewis tended to bowl short of a length”

Length (noun)
(in bridge or whist) the number of cards of a suit held in one’s hand, especially when five or more
“to open one heart with equal length in hearts and spades is in the modern Acol style”

Furlong (noun)
an eighth of a mile, 220 yards.