Ago / Back – They are used when we talk about the time/period count back from the present.

• I met Rakesh 2 days ago (back).
• He called me 5 minutes ago (back).
• My brother was playing chess a long ago (back).
• I have reached home just a while ago (back).
• A week ago (back), he had visited my house.

Before / Earlier – They are used when we talk about the time/period count back from the past/future.

• My salary was paid 10 days before (earlier than) yours.
• I had written the email before (earlier than) she apologized.
• I had spoken with him just a minute before (earlier than) I went to take a bath.
• He will meet me once again before (earlier than) 20th of the next month.
• I will complete the homework before (earlier than) the expected date.