One person on another forum said the CL-51 has twice as much ink as the CL-41. All I could find after much searching was that in some sellers descriptions (the others had no number of pages listed, not a different one) the CL-51 was rated to print 330 pages and the CL-41 was rated to print 155 pages. I figure that matches very close with double the ink.
The Canon CL41 is an ink cartridge. It allows one to print documents, letters etc in fine quality and color with sharp, smooth graphics. The cartridges can be refilled.