The semicolon, ;, is the far look command. It is used to look at a square other than the one you are standing on. To look at the square where you are standing, press :. Far look does not allow you to see what is actually on that square, only what you think you see. This means you won't get information about undetected monsters (or traps), and if an item has been moved since you last had line of sight to the area, you can still use far look on that item's symbol. Because of this, far look generally gives more accurate information about something when you have direct line of sight.
Using far look can help identify what a particular ASCII symbol represents, and will often give useful information about specific objects you select (especially creatures). This can help determine if a monster is peaceful or tame, for instance. In the case of monsters that are beyond your normal field of vision, but still detected, it will tell you what detection method is being used (like telepathy or infravision). You can also tell if you have called a creature by a particular name, which is handy for sorting out similar pets.
When you press ;, you will be prompted to "pick an object." Pressing ? gives the following advice:
Use  to move the cursor to an unknown object. Use [HJKL] to move the cursor 8 units at a time. Or enter a background symbol (ex. <). Type a . or , or ; or : when you are at the right place.
Note that you can enter a background symbol. This can be helpful in locating a feature such as a stairway or altar that may be covered by some other object. If the location is already known, pressing > would jump the cursor to the downstair.
Choosing a location gives information about the symbol there, such as:
+ a spellbook or closed door (closed door)
... versus ...
+ a spellbook or closed door (tan spellbook)
... or perhaps ...
d a dog or other canine (tame dog called Fido)
... compared to ...
d a dog or other canine (jackal) [seen:normal vision, infravision]
The what is command, /, makes use of the same cursor select mechanic as far look.