As per definition Nostalgia describes a longing for the past, often idealized and unrealistic.
The term was originally coined by Johannes Hofer, a medical student in 1678 (νόστος = nostos = returning home, άλγος = algos = pain/longing) roots, to refer to "the pain a sick person feels because he wishes to return to his native land, and fears never to see it again".
But as arimanius pointed out, nostalgia means a lot more in its current usage. This word is used in context of thinking about the past. Some good memories, some bad, lots of images swirling around in your head... making u smile, shake ur head in wonder, sometimes making u grimace... all memories, all yours...
I don't know abut people in general, but i could do away with some of these memories... but i guess those are the hardest to get rid of. It's amazing how i can quickly forget the small things in life that gave me pleasure, those little moments in time which are so important that they should be kept framed forever. I really don't remember the first times of so many things... the first day of school, the first day out of home all alone, the first day in a new city, the first day with a set of entirely new people... I guess they happen so quickly that there isn't time to take it all in. But the hard-to-get-rid-of memories... they are always the bad ones. All the mistakes you've ever done, all the embarrasing situations you've been in will come to haunt you the moment you sit alone. They come unbidden, almost in a sneaky way... and before you know it, you are living all those horrors again.
People learn from their mistakes, i don't seem to be of that breed... i keep on making the same mistakes over and over again. adding to my ever increasing stock of bad memories... someday they'll push out all the good ones :-(
The point is, i'm not really fond of this "Nostalgia" thing and i really wish out alcom finds a different name soon... not that i'll come even if they do that...