A basic template of an x-sheet comprises of a table with several rows and columns. Each row represents one frame of animation. A classic x-sheet holds up to 4 seconds of animation, i.e. 96 rows.
(Many classic x-sheets also have markings for Footage, i.e. 6 feet of the film reel, and 1 foot= 16 frames.)

The sheet then has the following columns:

Action
This column holds the timing planned out for the scene, how long the scene should take; and also the action of the character, at what point would the character have a particular pose.
Some animators also annotate the key and breakdown poses within this column to make the action clearer.

Dial
The dial holds the breakdown of the pre-recorded dialogue (or the beats of the music in case of no dialogue) to know at which frame to hit a certain phoneme.

Cel Levels
There are generally five of those columns, each representing one layer of cel, meaning to say one x-sheet can hold up to 5 layers of animation, although these many are never usually required.

Background
This column has the background numbers and for how long to hold one and when to switch to another.

Camera
The commands for the camera are put down in this column. If you want the camera to pan, or zoom in or just shake, all of that goes in here.

Scene, Sequence and Sheet numbers
The respective numbers are to be mentioned here.