Yes, Sleep Mode in Windows XP, Vista and Windows 7 is different from Hibernate feature which Windows provides. Both are means of temporarily suspending the functioning of your PC / Computer / Laptop, but there is a huge difference in the way each of them work in the background:
The difference is the way in which the program, data and state of the program is stored an retained.
In Sleep mode, the memory (RAM) of the system is used to store the state or data of the program.
In Hibernate mode, the state / data of a program that is running is saved to the hard drive of the system / computer / laptop.
The way the data is stored and retrieved later on plays an important role in deciding which feature you’d want to use.
Related reading:
How to hibernate in windows XP