What's the actual purpose of terrorist attacks? I really don't get it, what's the reason for killing multiple people because of their religion?

Many Muslim terrorists are convinced they are dying a martyr and will be instantly rewarded in paradise. They believe things such as that the killing of Westerners is part of the struggle for a worldwide caliphate (in the cases of ISIS) or that killing Westerners is revenge for Western intervention in the Middle East, but either way they are dying for a godly cause and eliminating non-Muslims from the world.

It's a power thing. Regardless of the stated motivation (muslim, christian, anti abortion, homophobe etc) at the core of it all is an all consuming narcissistic belief that they are the only ones who are right, and everyone who does not share that belief can only be convince otherwise by force.
And at the base of all that is a sad desperate need to control other people (usually due to a perceived lack of control of their own lives) - if you set off a bomb that makes people scared to go about their lives you are in a sense controlling them.

It involves enticing fear into people so they will submit.

Depends on the specific group. In the case of Islamic terrorism, the goal is largely to alienate muslim populations in western nations and make them easier to radicalize and make it more difficult for muslims to leave their native countries. The reason they want this is because for some time now, there's been a brain drain in the Islamic world. A large portion of their best and brightest leave their nations, and so with them goes wealth. So middle eastern nations fund terrorist groups who subversively make it harder for muslims to leave. They also fund terrorist groups as part of their wars against each other. Iran and Saudi Arabia have been fighting a proxy war for quite a while now. Sure the Koran is used as a tool to gaff up willing fools to fight these battles, same way Mohammed used it, and same way the Roman Catholic Church used the bible in the middle ages. Religion is one way to get desperate people to do things that seem unthinkably stupid.

The tactics they've been using have largely been successful too. That's why we're seeing more and more of it.

Do you think they're killing people "because of their religion"? These are random atracks and the victims are of different religions, races and beliefs.

There is no sense trying to make sense of terrorism.

According to their religion, those who do not follow the will of Allah are infidels that should be erased from the planet. And they believe that killing themselves for this cause will reserve them a place in heaven where 72 virgins will be waiting for them

I presume the purpose is to create enough fear so as to make it easy to control us.

virgins in heaven

I am from a Non Muslim from a Muslim country.When I was 12, a Muslim teacher told my class especially to Muslim students that dying for Islam and Allah is the most purest sacrifice any Muslims can make and they definitely will go to heaven.

they don't regard people as people basically. they want to take over the world and kill everyone who isn't of their faith

they have no regard for other beings.. basically

they are following the commands of the Koran, their holy book. they think they will be rewarded in heaven. it's pretty simple.

They believe they are doing gods work and that they are killing the bad guys...

DUDE
that's what i keep asking too

I don't understand it either.

its revenge, nothing more, they dont want 'anything' , its revenge for usa and uk supporting iraq et al in gisis, and s mosul is about to be liberated completely from isis, its their last dying breath, the muslim world hates isis, the soldiers fighting and killing isis are all muslims.

They probably feel they leave a better world.

They want a piece of the pie, it's all about power