There are hundreds of different creatures in the pits of Angband, many of which have the same letter symbol and color on the screen. The exact species of a creature can be discovered by looking at it. It is also very difficult to keep track of the capabilities of various creatures. Luckily, Angband automatically keeps track of your experiences with a particular creature. This feature is called the monster memory. Your monster memory recalls the particular attacks of each creature (whether or not technically a monster) which you have suffered, as well as recalling if you have observed them to multiply or move erratically, or drop treasure, etc. Otherwise you would simply have to take notes, which is an unnecessary bother.
If you have killed enough of a particular creature, or suffered enough attacks, recalling the monster memory may also provide you with information not otherwise available, such as an armor class, hit dice, spell types, frequency of spell casting, or the amount of damage for breaths or spells. These attacks will be color coded to inform you of whether or not you currently resist a specific attack. Red or orange means you do not resist it, yellow means you partially resist it, and green means you resist it or are immune. If you attack a monster with specific elemental attacks you will learn if the monster resists that element or if they are immune. There are other magical means to learn about monster’s abilities that don’t require you to actually experience the attacks.
This memory can be used by all your characters; it is stored in a file called ‘lore.txt’ in your user directory (~/.angband/Angband in Linux, lib/user in Windows, Documents/Angband in macOS).