They were added in the 19th century by Viollet-le-Duc, a French architect who undertook the restoration of many medieval churches. He added the Gargoyles to make the cathedral look more 'medieval' (though his idea of what looked medieval was very influenced by his contemporary, 19th-century context - he actually did a lot of damage to the original architecture to 'clean it up', ie get rid of the bits that didn't fit his notions of proper medieval art).

Here's an article that summarizes a good book on the subject. The article is helpful (and answers most of your questions), and the book is even more so if you want to delve deeper into the subject.

1. As far as I know it's a little more complicated than that and you need to distinguish between two things: gargoyles and chimeras. Gargoyles are essentially decorated end of drainpipes, whereas chimeras only have decorative purposes (no drainpipe or anything behind them, they're just normal sculptures if you like). In the late 12th century, when they started building it under Maurice de Sully, Notre Dame didn't have either, because gargoyles simply didn't exist at the time. Gargoyles were invented in the early 13th century when architects worked out that it was a lot better for the building if you could control the water falling of the roofs rather than having it pour directly onto whatever is underneath as otherwise it would damage it pretty quickly. Considering the sheer size of Notre Dame, this was especially true and architects had to come up with a drainpipe system to prevent rain water from damaging the different roofs and sides of the building, and they created gargoyles at the end of the drainpipes. In the early days, architects favoured few gargoyles in strategic locations on the buildings (true for Notre Dame and other cathedrals built around the same time) but then they found out that the more, the better, so they started making thinner, more decorative gargoyles, and added more of them. So there have been gargoyles on Notre Dame not quite since it was first constructed, but nearly (apparently the first gargoyles were added to the centre part towards 1225) However as explained by alriclofgar, most gargoyles were added in the 19th century by Viollet le Duc and Lassus when they did some extensive refurbishment of the cathedral. They also added all the chimeras, which incidently are now more famous than the gargoyles (ie most pictures you can find of gargoyles online are actually chimeras)

2.They're made of liais-cliquard, a sort of limestone found near the river Seine which is extremely difficult to erode (rather convenient for something meant to have water run through it all the time)

3.most of them from Viollet le Duc and Lassus in the 19th century, some from architect nb 4 in the late 1220s (can't find his name)

4.Yes

5.Sort of. They're supposed guard churches and scare evil spirits away from them. Also they might have served as a reminder to people that salvation is only available inside the church and warn people of dangers and temptation on the outside. Might be a few other theories, too.

source: Dictionnaire de l'Architecture, Viollet le Duc. Here's is an excerpt: http://agora.qc.ca/documents/art_gothique--gargouille_par_eugene_viollet_le_duc. Sorry, all in French