Rummy 500 is a member of the "Rummy" family of card games, more formally known as "Five Hundred Rum". Thirteen cards are dealt to you and thirteen to your opponent. The remaining cards form the stock and are placed face down by the table. The top card is turned face up on the table. This card forms the beginning of the discard pile. The game progresses with the player and opponent alternately starting their turn by taking either the top card of the stock, or one or more cards from the discard pile.
You may always take the last card of the discard pile as an alternative to taking a new card from the stock, unless the rule variation inhibiting this is in effect. You may elect to take another card from the discard pile only if you are able to meld the card you select. The selected card, and any and all cards to the right of it, are added to your hand. Your next action must be to meld the selected card.
Whether you started your turn by taking a card from the stock, or by selecting cards from the discard pile, you continue your turn by playing melds to the table as many times as you wish - providing that you have the cards to meld. You conclude your turn by discarding one of the cards in your hand, unless you have melded them all.