You probably already know what a CSV is, but a little refresher won't hurt.
The idea of the format is to take a table of values and write all rows down as records. Inside a record, every column item is written down and separated by a comma. That's where the format's name comes from—comma-separated values.
Let's do an example. In the following code, we are going to write a CSV comparing various planets in the solar system to our own. A radius, distance from the sun, and gravity of 1 means exactly as on earth. Written as a table, our values look like this:
| name | radius | distance_from_sun | gravity |
| Mercury | 0.38 | 0.47 | 0.38 |
| Venus | 0.95 | 0.73 | 0.9 |
| Earth | 1 | 1 | 1 |
| Mars | 0.53 | 1.67 | 0.38 |
| Jupiter | 11.21 | 5.46 | 2.53 |
| Saturn | 9.45 | 10.12 | 1.07 |
| Uranus | 4.01 | 20.11 | 0.89 |
| Neptune | 3.88 | 30.33 | 1.14 |
Take every row, separate the values by commas, put them each on a separate line, and you end up with the CSV file:
name,radius,distance_from_sun,gravity
Mercury,0.38,0.47,0.38
Venus,0.95,0.73,0.9
Earth,1,1,1
Mars,0.53,1.67,0.38
Jupiter,11.21,5.46,2.53
Saturn,9.45,10.12,1.07
Uranus,4.01,20.11,0.89
Neptune,3.88,30.33,1.14
As you can see, the heading ( planet,radius,distance_from_sun,gravity) is simply written as the first record.