Filter circulation is water that moves from the tank to the filter system and back to the tank.
Internal circulation is the water that moves within the tank itself.
There are many methods and theories for accomplishing both. Personally I believe lack of good Internal Circulation is one of the main reasons for algae problems. Available are an array of devices to accomplish some of these types of movement. Oscillators, there are a few manufactures who produce units that either move your powerhead back and forth 90 degree's, or units with motorized water discharges that move back and forth 90 degree's. Dump Buckets, an offset bucket that fills and then tips to dump its water, then rights itself to refill.