Mucology, or dealing with wild mushrooms.

When lost in a forest or during other similar situations where you need to forage for food really fast to avoid starving and you're unable to hunt or gather fruit, wild mushrooms can be a good source of nutrients but also a very risky one.
In short: If you don't know your left from your right about mushrooms, just don't forage for mushrooms at all. Leave it, do something else.

But since we have the time and luxury to go through mushroom basics here, let's give it a go:

First of all not every inedible mushroom is poisonous, most of them are impalatable, bitter or too tough to eat.

Rule no1 is "Only focus on mushrooms with a very recognisable form, stick with mushrooms you're absolutely certain about". Many species of mushrooms might look like other (edible) kinds of mushrooms in their early form, so it's likely that the inexperienced eye might not be able to tell those two apart. Therefore even seasoned mushroom enthusiasts keep a booklet at arm's length.

When identifying a mushroom you're looking for certain features:
-Where is it growing? (on the ground or on a tree, or maybe inside the earth)
-Is it in groups, or cluster, or alone?
-Particular form or shape (straight or bulbus)
-Colours (could be any colour of the palet really)
-Whether it has a stem & cap, and whether the cap's underneath has gills.
-Texture of the mushroom (leathery, fleshy, crumbly, smooth, hairy, or slimy)
-Their scent/odour (I'm not kidding, most mushrooms have a very specific scent)

Before we go into detail, a small rule of thumb is that mushrooms with white gills under the cap are inedible. There are of course some exceptions but (in the western hemisphere) most white gilled mushrooms we find are the Amanita muscaria species which are extremely lethal.