Sealed for the Day of Redemption

Most of us are familiar with the Good Housekeeping Seal of Approval as it applies to manufactured goods. What did Paul mean in Ephesians 1:13 when he said that believers were sealed with the Holy Spirit of promise? All of us are familiar with what it means to grieve or to be grieved by someone else. What did Paul mean in Ephesians 4:30 when he said do not grieve the Holy Spirit of God?