Font commercial license?

I am a bit confuse what is font commercial license for?

Do I need commercial license under this circumstances?

1. 
   

   designing a banner for small client.

   
   


2. 
   

   Using a font and then alter its shapes into a logo for client.

   
   

Also after I purchase the license does that mean I may need to purchase it for my client as well? (even just a small job like designing a banner.)