1) Unless the blocking group contains creatures with Banding (which is an ancient ability in Magic), the attacker assigns damage to the creatures. With the rules changes with Magic 2010's release, it works like this:
--Immediately after all Blockers are declared, the attacker determines the priority in which damage will be dealt to the blockers by the attacker.
--When damage is dealt, The attacker must assign damage in the order that they determined the priority earlier. Each creature must be assigned at least lethal damage (damage = to its toughness, or 1 damage if the attacker has Deathtouch) before any damage is assigned to another creature. If the attacker has Trample, any Power left on the attacking creature after lethal damage is dealt to all Blockers may be assigned to the defending player/Planeswalker.
2) Cascade works like this: When a card with Cascade is put on the stack, reveal cards from the top of your library until you reveal a non-land card with a converted mana cost lower than the cascade card's converted mana cost. You may then cast that card without paying its mana cost. Put all cards revealed by Cascade and not cast on the bottom of your library.
One of the biggest advantages for Cascade is that you get to cast two cards for the mana cost of one. This makes it far more difficult for control decks to handle. Also, Cascade can pull another Cascade card, so you can potentially get something like Bituminous Blast + Bloodbraid Elves + Violent Outburst + Putrid Leech all cast just for 5 mana. Alternately, I've used it in a deck to have a huge number of potential draws to get to Rampant Growth by making Rampant Growth the only 2 mana card in my deck, and make all the 3-mana cards and some of the 4-mana cards have Cascade. So every time I play a Cascade card, I'm guaranteed to get Rampant Growth out of it.
However, you have to be careful how you use it. If you were to toss a bunch of Cascade cards in a deck that was built around a card that had a higher casting cost that the Cascade cards (and that card is one of the few with a higher casting cost), you'd practically be guaranteeing you wouldn't draw that key card, as the Cascade cards would be likely to put it on the bottom of your deck.
3) Tokens are permanents that do not have an actual card to represent them. Frequently, people will use regular playing cards as the token creature representative. "Saproling" is just the creature type of the token, which is only significant with the abilities of certain other cards (like Coat of Arms).
4) Yes. Summoning sickness only prevents attacking and using an ability on it that causes it to tap. It can still be tapped by other abilities, though.
5) The color(s) of a card are determined by the colors of mana that are required to by paid to play it via its casting cost. So a multi-color card is a card that has more than one actual color of mana in its casting cost.