They are lines on the screen, so you can see where the spreadsheet cells are located. You can see them when you edit, but they do not appear when printed, unless you enable their printing. Usually, you add cell borders if you want the lines to show up when you print.
The lines on the spreadsheet which are call gridlines shows each individual cell
They are called gridlines and should not be confused with borders, which you can draw around cells. Gridlines can be switched off by going to the Tools menu and picking Options and under the View tab removing the tick beside Gridlines.
The worksheet.
grid
You can create a form with MS Excel that has both horizontal and vertical gridlines, you just format the borders of the cells you want to display.
When you look at a worksheet in Excel you can see all the cells, because of the lines around them. Those are gridlines. You can also add borders, to make heavier lines around cells. Gridlines can be turned off completely, by going to the Options menu and the View section and removing the tick beside Gridlines.
There is no single answer to that as there are different chart types and you can specify what way you want gridlines to be shown, like the gap in terms of value between them. You can also set a chart to have no gridlines.
gridlines
Gridlines on a chart indicate values. You will see them on bar charts, column charts and others. Major gridlines show more significant values, like having a major gridline to indicate intervals of 10. So there would be on at 10, one at 20, one at 30 and so on. You can also have minor gridlines which will be thinner and could be at more frequent intervals.
Gridlines
You apply borders to some or all cells. Don't confuse borders with gridlines. Gridlines show the cells. A new worksheet has no borders, just gridlines. You can then put borders around whichever cells you want or just around parts of a cell, so not all 4 sides. Gridlines can be turned off to give more effect to borders.
They are separated by gridlines, although the gridlines can be turned off. EAch column is identified by a different letter or combination of letters.