There is two types of Humulin. R and N. Both will have "U" after them.

The U simply indicates the "concentration" as in U-100, U-200, U-500 etc Where U-500 is 5 times as concentrated as U-100.

The difference between the N and the R is how fast they work and how long they last.

Humulin R is a quicker acting insulin - starts working in 30 minutes, peaks in 2 to 3 hours and lasts about 8 hours total.

Humulin N is slower acting - starts working in 2 to 4 hours, peaks in 4 to 12 hours and lasts up to 18 hours. This would more typically be used as a "night time" insulin to help with fasting numbers.