Secant and Tangent lines

So I don't really understand the difference between a Secant line and a Tangent line. 
What is the difference between them?
When are they supposed to be used/ can be used? 

What do they represent? 
How do you know if a line is tangent or secant? 

If somebody can clarify these for me i'd really appreciate it! thank you :')


Also, 
does anybody know what an average rate of change is? :( 

i'm sorry if i keep asking questions i just don't understand any of this lol