Shouting is simply speaking loudly or making a loud sound. If you saw your friend some distance away at a crowded park, you might shout her name to get her attention. A soccer coach might shout instructions to the players on the field. Shouting can be angry, joyful, or neutral.
When you scream, you make a long, loud, high-pitched sound. People usually scream in moments of fear or pain. It is often a reflex action (screaming when something frightens you) but you can also scream intentionally. A parent whose small child starts to wander out into the street might scream for him to stop. People who are trapped in a car after an accident might scream for help.
American beer being weak as piss stems from the fact that most countries measure beer alcohol percentage by volume. The U.S., continuing our proud tradition of shunning the rest of the worlds measuring systems (just like that metric system bullshit), has traditionally measured alcohol exclusively by weight instead.