Defending a hand in bridge is the same as declarer play in the sense that you're simply trying to use your good cards effectively to win as many tricks tricks as possible, establish your long suits and score extra tricks with your trumps.
The defence need to work together so if your partner has led a suit then it's often going to be best to keep leading that same suit should either defender regain the lead.
On the following hand West is declarer in a contract of 3NT and North, your partner, leads the ♥Q. If you are able to win a trick, maybe with the ♣A, you'll know to return partner's suit rather than take your ♠A too soon.
Return partner's suit!
If trumps are breaking badly for declarer then it can sometimes be worthwhile forcing declarer to use trumps leaving you in control.
You lead the ♠A then the ♠K against 4♥ contract. East ruffs the second round and plays the ♥K. What now?
Force declarer to ruff
Winning the ♥A then playing another spade will force declarer to ruff and you'll end up with more trumps than East!.
Sometimes you may be able to foil declarer's plan to win extra trump tricks by ruffing by playing trumps yourself.
On the following hand, North leads a diamond against West's 4♥ contract. You win the first round of diamonds and immediately switch to a heart.
Play trumps to prevent ruffs in the dummy
If you can remove the two trumps from dummy you might be able to win three or even four diamond tricks.
Sometimes you're defending a hand and you can see that declarer's finesses are going to fail and that nothing is breaking well. Perhaps your opponents appear reluctant to bid game. Passive defence gives nothing away.
Time to go passive!
Partner leads a spade, the unbid suit, we win and switch to another suit giving West nothing by doing so.