In-Circuit Programming means literally what it says: the device can be programmed in situ without having to be removed and placed in a separate programming unit; however, a special programming connection (eg, JTAG port) may be required.

In-System Programming, I think, means that the in-circuit programming can be done through the system's normal "user" interface(s); like when you download new firmware to a modem via its normal RS232 port.

That's my understanding, anyway.