Puff pastry is made from a rolled in dough
(that is fat is rolled in
between the layers of dough) and is considered a rich dough product. It is similar to danish dough
except that it is leavened by steam alone and does not contain any yeast.
It is used for making flaky, crusty bread products similar to your croissant. It can be used to make turnovers and can be substituted for danish dough if one is inclined. The rolling in process is simple enough but is easier understood through illustrations.
It is not used for strudels as strudel dough is basically a phyllo dough, a very thin stretched out dough.
The challenge with making high quality puff pastry is in the rolling in of the fat. Preferably this fat is the kind with a high percentage of fat (close to 99% fat). This poses a problem as butter is hard when too cold, and liquid when warm. To be able to pass the puff pastry through a dough sheeter while maintaining the butter in the dough at the just right temperature to keep it plastic is a bit tricky and either means you work fast or you do the four folds in stages, returning the dough to the refrigerator every time you fold. Some bakers use a puff pastry margarine which does not pose those problems but the product one gets with these margarines are inferior as they do not "melt in your mouth", that is they have a melting point higher than human body temperature. Thus it leaves the mouth with a coating of congealed fat and is not very pleasant.
For those who are not willing to do the work of making puff pastry but want to use it in their cooking, there are many commercial made puff pastry which actually use high fat butter and are sold in frozen sheets. These actually produce a better puff pastry product than home made puff dough will if one is not yet too familiar with the rolling in of dough.