A heatercore is a radiator, only smaller, and the water makes two passes instead of one.
A car's main radiator (in most cases) passes water from one endtank and exits at the other, passing through all the fin area once.
A car's heatercore water passes through half of the fin area to a common tank on the other end, then passes back through the second half of the fin area to exit on the same end of the radiator.
Two passes through doesn't make that much difference, but it does allow a smaller size, and only one fan to cool it.
There are other types of radiators though. A serpentine tube style is one example where the water passes through one tube, making passes back and forth. These are a tad less efficient because they restrict flow because of the many turns the water makes, however they are made to take much more pressure, so they have their uses. Like transmission coolers, and condensors/evaporators for air conditioning. They also are harder to clog up as the passage through it is larger....great for dirty tranny fluid.
That about cover it?