Cont'd -
Halloween is, as pointed out, a derivation of the Celtic Pagan Samhain. (Here in Ireland, it's pronounced sow-en, sow being the flat 'o' like ow when you bang your skull getting into the car heading to work, but that's another story.). In fact, Samhain is the word still used for November in Irish.
As Pink Kitty has pointed out, this is one of the four major sabbats in the year, (of eight - 4 major and 4 minor). It's the time where the veil between the worlds is seen as being at its thinnest. Food and a candle (hence the jack-a-lantern) are left for wandering souls. In modern Pagan tradition, (Wicca, Druidry, Asatru, etc) the sun-god dies on that night and is reborn at the midwinter solstice.
It marks the beginning of Winter and the start of the new year.
It's a much celebrated holiday and is huge here in Ireland, and in fact we exported it to America with the thousands of emigrants. It's obviously changed since then, but us Pagans do like a good party so we welcome it as the major festival of all.
We in Ireland celebrate with parties and games. There's a national bank holiday too (on the nearest Monday in October). Traditional food is brack and beer. Also, we love the trick or treat thingy - it's not just a US thing, it never died out here.
Basically, as with all religious festivals, it's a time for reflection and celebration, and long may it be so. Oh, and as usual, everyone's welcome, regardless of faith or creed.