Upper bounds and Lower bounds in Algorithms

I saw several articles describing upper bound as Best Case and Lower bound as Worst Case. Meanwhile some articles have given explanations for Upper /Lower bound of Worst Case.

So basically this got me asking three questions:

1. What exactly are Upper/Lower bounds?
2. How can they be defined separately in a Worst Case scenario?
3. Can bounds be defined for other cases(Best,Average) as well?