BGA is integrated circuit packaging standard based on PGA where the pins are small balls. It is soldered to the printed circuit board through SMT technology. This kind of packaging is used by motherboard chipsets and some memory chips used by video cards. Is also known as PBGA (Plastic Ball Grid Array).
click to enlarge Figure 1: BGA packaging.
click to enlarge Figure 2: Example of a chip using BGA packaging (motherboard chipset).
click to enlarge Figure 3: Another example of a chip using BGA packaging (video memory).