Go with the Flow

The notion of card advantage is not exclusive to Yu-Gi-Oh! Having more resources than your opponent can be very beneficial in a number of games and even in war. More cards means more options and that can be critical in determining which side will be victorious. However, more does not always equal better. If while dueling someone, you find yourself with six Gadgets in hand while your opponent has one Judgment Dragon on their field, the situation is not exactly in your favor. Therefore it is very important that you make optimal use of the resource you have available. Consider the following scenario:

You summon Elemental Hero Neos Alius and use it to attack your opponent's face-down monster which is revealed to be Skelengel. Your opponent draws 1 card due to its effect. While you might have gained field presence over your opponent, the Skelengel you destroyed has already replaced itself with another card. Cards such as these—known as "floaters"—allow players to make moves without risking a loss of advantage. Look at it this way:

You destroyed a Skelengel by battle. (-1 for your opponent)
Your opponent draws a card due to Skelengel's effect (+1 for your opponent)
Net card advantage is: -1 + 1 = 0

The Gadgets are a prime example of floater concept as each of the three Gadgets (Red, Green, and Yellow) adds another to your hand, so that even if your opponent destroys it, your card has already paid for itself. It is for this reason that cards like Deep Sea Diva, Elemental Hero Stratos, and Gravekeeper's Spy are widely used.

Note that Monsters need not have effects to be considered floaters. If a Normal Monster were to destroy at least one of your opponent's monsters by battle then it would become a floater.

But as I said earlier, having more cards than your opponent isn't everything. What's important is that you utilize what resources you have optimally. Let's compare two very popular monster removal cards:

Smashing Ground:

Destroy the 1 face-up monster your opponent controls that has the highest DEF. (If it's a tie, you get to choose.)

Lightning Vortex:

Discard 1 card. Destroy all face-up monsters your opponent controls.

While both cards destroy your opponent's face-up monsters, the former is costless and affects only a single whereas the latter requires a discard and can destroy as many as five. Because of this using the Lightning Vortex to destroy only one of your opponent's monsters would not be an optimal play as the former could same without the added requirement (the discard).

Using Lightning Vortex while your opponent has 1 face-up Monster:
-1 (the discard)
-1 (the Lightning Vortex itself)
+1 (for the opponent's monster you destroy)
Net card advantage is: -1 + -1 + 1 = -1

Using Lightning Vortex while your opponent has 2 face-up Monsters:
-1 (the discard)
-1 (the Lightning Vortex itself)
+2 (for the opponent's monster you destroy)
Net card advantage is: -1 + -1 + 2 = 0

Using Lightning Vortex while your opponent has 3 face-up Monsters:
-1 (the discard)
-1 (the Lightning Vortex itself)
+3 (for the opponent's monster you destroy)
Net card advantage is: -1 + -1 + 3 = +1

That's not to say that -1 yourself isn't always an acceptable move but it is not always likely to be advantageous in the long run to the players who does.

Card advantage may sound pretty straightforward, but it's up to the player to know when to respond appropriately. After all it's better to -1 yourself to by using Lightning Vortex to destroy your opponent's Dark Armed Dragon when he/she has two more Dark Monsters in his/her Graveyard than simply want for him/her to summon more monsters. With enough experience any player can make the right call.

Remember players, the game ends with you.