One unit of weight in Cataclysm represents about a quarter-pound (or 100 g).
Small food-items have a weight of 1 unit. So do bottles of water and other containers. Always choose the largest container for the least amount of overhead. If you have the Pack-mule trait then Jerrycans of clean water will become very attractive in a longer game.
Bandages, Drugs and Medicines weigh 0.01 units. This is rounded down on a per item basis so that you can carry multiple stacks of 99 of these items without fear of overburdening your self.
A character in Cataclysm can carry a certain amount of weight based on the character's strength. The limit is equal to 170 + 9*(strength - 8). If the character's strength changes because of drugs, pain, or other effects the carrying capacity is adjusted accordingly.
Characters can carry much more weight than this capacity indicates but they will become Overburdened and suffer pain and encumbrance penalties, greatly lowering their speed and accuracy. Remember to check your stats and to drop excess weight before entering combat.
Generally speaking you will run into volume issues more than weight, but unlike weight you can't craft a makeshift sling to carry more. Prioritize when you are carrying too much. Leave books and non-perishables behind. Leaving perishables may mean they will rot before you get back, and no point in using up your canned food if you have fresh alternatives. While we may want to be a walking library, books won't feed you, heal you or protect you. Make a ote of the dropped items on your ap. Come back for them later.