It looks much like a large microprocessor chip, not much bigger, but sealed inside are enough components - processor, memory, power supply, antenna, lens, sensor, etc., to transform the package into something very powerful. Introduced in 1998, the "stacked chip-scale package" was a big advance over the printed circuit board. A standard circuit board spread the functions out which slowed down performance. The SiP are naked chips stacked up vertically and interconnected within a single very compact package. Imagine the standard circuit board as suburban sprawl while a SiP is a highrise :-)
The idea of the "Silicon burrito" comes from the fact that it uses a flexible tape that rolls up the chips to create a very compact method to pact additional elements - resistors, capacitors, inductors - into the same high-rise (or burrito). All of these designs shorten wires, boost speed, and improve the overall performance. This system specifically helps cell phones and digital cameras.