Clash-of-clans – Clash of Clans crashes when I click the Donate Troops button

If you are in a clan, and other members of your clan donate troops to you, they will be stored in your clan castle and come out for defense. This is the only way that I know of to defend with troops. So no, you can't train troops and use them for defense. However, others may donate troops that you can use for defense. Also, skeleton traps may be considered troops by some, even though they are not trained.

EXP in Clash of Clans is pretty much like experience in any other game.
It can be found on the top left-hand corner of the main screen and profile screen.

The experience you gain then is placed alongside a level system, with EXP cut-offs, a number that increases as you increase your EXP, but the requirements to get to the next level also increases.

It basically shows how much a player has invested into a game. Or for most MMORPGs; their character.

You pretty much earn experience by:

• Upgrading Buildings
• Donating Troops
• Earning Achievements.
• Destroying townhalls in attacks
• Clearing plants/rocks from your village.

For all we know, there could be a Level 30 player with a Townhall Level 8 in Challenger League amongst all the Level 90+ players.

Answering your question:

It basically shows how much a player have invested into a game.

The only thing EXP is useful for is for unlocking decorations.
The more EXP you have, thus giving you a higher level, the more decorations you unlock.

If you need more information, you can find them in the Wiki:
http://clashofclans.wikia.com/wiki/Experience
http://clashofclans.wikia.com/wiki/Decorations