The paint format tool in Google documents lets you copy the formatting you have applied to a specific section of text to another section.
The Format paint icon, located on the toolbar next to the web clipboard, allows you to set one area of text to the format you want (font style, size, type, colour, bold, italics etc) and copy that format to other areas of text. Double clicking the format paint icon will lock the paint - allowing you to select multiple areas of text to be changed.
This is a very effective method of speeding up the formatting of lines of text, but it is also useful when working within a table in a Google doc. Whilst you cannot select a column in a table, eg to quickly bold all text in a column, you can use the format paint tool to speed things up. Simply format the text in one cell as you require and then use the format paint button to copy that formatting to any other cell.
Another useful feature, if you have the paint tool locked - using the Ctrl-Shift-V action to paste will paste text formatted in the style last locked. To unlock the paint tool click the icon.
You can also use the paint format tool with keyboard shortcuts. Ctrl-Option-C will copy the formatting of the text you've selected, and Ctrl-Option-V will apply any copy that formatting to a different text selection.