Like others have said pre-eclampsia is the first stage (though still very serious) and eclampsia is full blown. The exact cause isn't known but it's basically the placenta causing the symptoms and so it can't be cured without getting the placenta - and hence baby - out. This isn't always the cure in itself and meds are sometimes needed too.
I had hellp syndrome which is a rare complication/form of pre-eclampsia /eclampsia. I had an emergency section as there wasn't time to induce, hellp is so threatening. I was on some really strong meds afterwards to treat the hellp, I had numerous drips and oxygen etc and had to stay in high dependency unit for a week. I survived to tell the tale though fortunately, fatality from hellp is quite high globally but virtually non-existent in UK luckily for me, I guess that's a sign of the importance of detection and medical care available. I had no idea how ill I was before the took my down to surgery, I kept saying "yes just do whatever is best for the baby", it was only afterwards I found out that Isobel's heartbeat was perfect and they were worried about me not her!