The first digit is the year model, as in 5 means 1965 and 6 means 1966
The second digit is the code for the plant at which your car was built. F is Dearborn, MI, R is San Jose, CA and T is Metuchan, NJ. Those were the only three plants that were building Mustangs then.
The third and fourth digits represent the body style. 07 is a coupe, 08 is a convertible and 09 is a fastback.
The fifth digit is the engine code.
U is a 170 cid six cylinder with a 1 bbl carb
T is a 200 cid six cylinder with a 1 bbl carb
F is 260 cid V8 with a 2 bbl carb
C is a 289 cid V8 with a 2bbl carb
D is a 289 cid V8 with a 480 cfm 4 bbl carb
A is a 289 cid V8 with a 480 cfm 4 bbl carb
K is a 289 cid V8 with a 600 cfm 4 bbl carb ( along with some other cool stuff)
The last six digits are the sequential production, or 'serial' number.