"Introduction to Tactics" introduces the student to the tactical tools of chess (forks, pins, etc.).
Silman Teaches Tactics - IM Jeremy Silman The classic challenges in this module contain combinations and tactical material for intermediate and expert level players (USCF or Elo ratings between 1400 and 2000+). Some challenges here are difficult even for masters and above. However, players of all levels can enjoy and learn from these examples.
"Step Up in Tactics" presents exercises, both checkmates and tactical, which are somewhat more difficult than the material covered so far.