What's this BGA chip and what's its purpose?

I opened a computer of one of my friends to see if something was wrong inside it. I'm not an hardware expert at all, in fact when I looked inside it, I thought one component was the CPU, because of its squared shape similar to a CPU, but apparently it isn't the CPU, since the CPU is usually under the cooler. (Note the cooler is below the graphics cards in red that you see in the picture below, i.e. the cooler is not visible in the picture)

This is a picture of the squared chip (I included other parts so that you can have a more natural feeling of its position and the relation with the other components):

I looked online for the name "SLJ48" (which is written on the chip in very tiny font) and something came out, for example:

but no clear explanation...

So, if that isn't the CPU, what the heck is that and what's the purpose of that chip?

2 Answers

As indicated by the eBay page, it's the H61 Chipset Platform Controller Hub. It contains the SATA and USB controllers and provides additional PCIe lanes, among other things.

This is probably a part of the chipset -- a north- or a south-bridge. This Wikipedia article explains it well.