They are very closely related. "Sliding Window" is a concept in "Windowing" and some times referred to as the same thing.
One definition, is were the receiving device can send a single acknowledgement message for multiple packets of data sent in one window. Within that acknowledgement message is information about the receiving device’s buffer size, which tells the sending device to increase or decrease the number of packets in the next transmission (this is where the "sliding" in the name comes in).
There is more to the word "slide" than what meats the eye, it seams that there are multiple different definitions. Check those References: