If you are unfamiliar with the game of Texas Holdem, please first read the article entitled Texas Holdem, then return here for specific distinctions of No Limit Texas Holdem.
In No Limit Texas Holdem, the blinds are predetermined at the beginning of the game and the minimum initial bet is equal to the bring-in, unless a player doesn't have that much, in which case they would simply go "all-in" (betting all their chips). All subsequent bets beyond the first in any betting round must be equal to or greater than the amount of the previous bet/raise, unless the player is going all-in. The maximum bet in any hand is restricted only by the amount of chips in front of the player currently betting. Therefore, any player may, at their turn, go all-in.
At any online No Limit Texas Holdem table, there is a maximum allowable buy-in, preventing a player with a sizable bankroll from forcing out the competition with a series of bullish all-ins.
There is no maximum number of raises in No Limit Texas Holdem. However, a player may only raise amounts equal to or greater than the previous bet or raise, unless going all-in. If a player raises to all-in with less than the minimum required raise, the next player still has the option of raising the minimum required amount. However if that player only calls, no subsequent players in that round of betting are permitted to raise.
A player may force opponents with shorter chip stacks to go all-in. If only one opponent calls the all-in, the amount actually bet would then be equal to the amount of chips held by the player with the shortest stack. If more than one player calls a bet of all-in, a side pot is created for those players who have more chips to risk. This side pot will contain any amount above and beyond that which the player or players with the shortest stack(s) can call. After the Showdown, a player can only win the pots they have called completely.
Player A has $600, Player B has $700, Player C has $700, Player D has $500. Player A is first to bet and goes all-in. Players B & C, to call, only have to post $600. If Players B and/or C lose, they still each have $100 in their stacks. Player D can only call $500.
Therefore, if all four players wish to remain in the hand, the original pot stays at $500 per player (totaling $2000) and a side pot is created for Players A, B, & C, containing $100 per player (totaling $300). Players A & D are now both all-in. All players are still dealt cards, but only Players B & C can continue to bet, and all their bets go into another side pot. If Player D wins the hand, he/she wins the original pot. The first side pot goes to whichever of Players A, B, and C has the second best hand. The second side pot goes to whichever of Players B and C has the better hand.