The 1-Up Mushroom is one of the many trademark Mario items. Ever since its first appearance in Super Mario Bros., its been in practically every major Mario game ever made. The 1-Up Mushroom is far by the most famous and recognizable 1-Up symbol in all the gaming universe.
1-Up Mushrooms come in many shapes and sizes, but the most common form of it is basically just a thick looking mushroom with a light-tannish stub with eyes and a green top with multiple white dots. It is also known though to be without a face like in the Mario & Luigi series, have a red top saying 1-Up like in Super Mario Bros. 2, and even have yellow dots like in Super Mario 64.
In the games including the Super Mario Bros. series, Super Mario World, Super Mario 64, Super Mario Sunshine and Super Mario Galaxy, they simply give Mario and co. an extra life, increasing the amount of times they can die before they get a Game Over, and are generally very difficult to find.
In the Mario & Luigi series, if one of the Bros. lose all of their HP and fall in a battle, another Bro. that is still alive can use a 1-Up Mushroom to bring the fallen Bro. back to life with 1/2 their total HP.
There are also many Mushrooms similar to 1-Up Mushrooms, such the 1-Up Super, of which in the Mario & Luigi series brings a bro back to life with full HP, unlike the normal Mario & Luigi version of the 1-Up Mushroom that only brings back 1/2 their total HP. There is also the Life Shroom, a pink colored version of a mushroom from the Paper Mario series of which automatically brings Mario back to life with 10 HP if he runs out of HP during battle and falls.