In this chapter, David fights and wins several more wars.
I don't have anything clever to say about this chapter. Israel is surrounded by hostile powers on nearly every side and has been involved in a string of conflicts going back almost continuously since the day that they marched into the lands east of the Jordan. They couldn't even make it into the promised land before they had to start fighting wars.
In this chapter he fights: the Philistines, Moab, Zobah, Ammon, Amalek, Aram and Edom.
Out of these nations, the only one we have not heard about before is Zobah, though it was briefly mentioned 1 Samuel 14:47 as being one of the kingdoms that Saul fought against. I took this opportunity to do some quick research on Zobah and it appears to have been an early city-kingdom associated closely with the Arameans, which fits well with the context in this chapter, because v. 5 says that the "Arameans of Damascus" went to help Hadadezer.
I don't think this is a very interesting chapter. The only real comment I have is to remark upon how violent these conflicts appear to be. David, for instance, executes 2/3rds of the Moabite captives by height, which is pretty brutal, but as I've discussed before, these are wars of survival. Israel (and these other nations) are fighting for survival, and as horrible as it sounds, if they did not kill a bunch of the Moabites now, the Moabites would just reorganize and attack Israel again later. I should mention, though, that while Israel has a long history of conflict with Moab, this is the country where David left his father and mother during the long years of conflict with Saul. David was distantly related to the Moabites because of his great grandmother Ruth. It's also possible that the king of Moab was trying to encourage David's insurrection against Saul to weaken Israel. Whatever the reason for their conflict, David's treatment of the Moabites is, ahem, ruthless.
All of these countries are also trying to establish themselves are regional powers. For a long time Israel was subservient to the Philistines, but now these other nations (like the Moabites and Edomites) are becoming subservient to Israel, bringing them tribute of gold and silver.
David hamstrings nearly all of the horses that he captures to fulfill the command in Deuteronomy 17:16, which effectively keeps Israel from building a large military so that they will always be dependent on the LORD.
Lastly, we are given a list of David's court officials. The most significant of these are Joab (who we know), Benaiah, who leads the "Kerethites and Pelethites", which are ambiguous terms referring to David's "mighty men", his elite heroes and chosen soldiers who gathered around him while he was still a renegade from Saul. The Kerethites and Pelethites are David's closest allies and become a big part of his military base of support. It's possible that they were drawn from specific tribes (see e.g. 1 Samuel 30:14, when David falsely says that he "raided the Negev of the Kerethites" - this implies the Kerethites may have been a tribe or clan allied with Israel).