What is the significance of taking hold of the horns of the altar? I was wondering why did Joab take hold of the horns of the altar in 1 Kings 2:28? Or rather, what is the significance of him doing that? What is he trying to do?

1 Kings 2:28-29 (NIV)
28 When the news reached Joab, who had conspired with Adonijah though not with Absalom, he fled to the tent of the LORD and took hold of the horns of the altar. 29 King Solomon was told that Joab had fled to the tent of the LORD and was beside the altar. Then Solomon ordered Benaiah son of Jehoiada, “Go, strike him down!”

And when Adonijah learned that Solomon was made king, he took hold of the horns of the altar :

1 Kings 1:49-51 (NIV)
49 At this, all Adonijah’s guests rose in alarm and dispersed. 50 But Adonijah, in fear of Solomon, went and took hold of the horns of the altar. 51 Then Solomon was told, “Adonijah is afraid of King Solomon and is clinging to the horns of the altar. He says, ‘Let King Solomon swear to me today that he will not put his servant to death with the sword.’”
This looks like a reference to the manslaughter provision in Exodus:


  Exodus 21:12-14
  
  12 He that smiteth a man, so that he die, shall be surely put to death.
  
  13 And if a man lie not in wait, but God deliver him into his hand; then I will appoint thee a place whither he shall flee.
  
  14 But if a man come presumptuously upon his neighbour, to slay him with guile; thou shalt take him from mine altar, that he may die.


By fleeing to the tabernacle and taking ahold of the altar, Joab was claiming that he was not guilty of murdering Absalom (see 2 Samuel 18) and deserved mercy.  Apparently Solomon disagreed.