Conjunctions connect two sentences or clauses. 
'Before', 'after', and 'while' are also conjunctions. 
'Before' indicates that something happens earlier than something else.
- I want to go to the store before it starts raining.
  - Before I took a shower, I cooked dinner.
 
'After' is the opposite of 'before'. 'After' indicates that an event takes place later than another event. 
- I'll go to the store after it stops raining.
 
'While' tells us that two actions happen at the same time. 
- I don't want to go to the store while (= at the same time as) it is raining.