In what situations am I required to license fonts from its original creator? For example, if I design a poster or a card for personal use and I use a font I have downloaded online or have paid for the use of it, do I have to license the font?
What if I am using the font for commercial purposes? (Websites, phone apps, …)
Uses and rights vary by creator, and you need to read the license that comes with the fonts you are using for details.
In general, you need a license for each person who is using a font and a license for the printer, though the latter varies even more than the former. Redistribution of a font for electronic purposes is still relatively new and may require special permission if not a separate license from the creator if not explicitly given in the agreement that was sent with the font files.
You’ll want to look at this question for a more detailed discussion of how it all works.