I have found that the rules for Dungeons and Dragons v3.5 regarding combat are not presented in a way that is easy for a beginner to understand. Below is my attempt to make the combat rules easer to understand.

Each round represents 6 seconds in the game world.

In a round, you can do one of the following things:

• Attack and move
• Move and attack
• Move and move again
• Perform a full-round action

-You may also perform any number of free actions (within reason) and take a 5 foot step (if you haven’t moved otherwise).

Attack

You can make an attack, cast a spell, or perform an equivalent action – also called a standard action. Some standard actions are: Aid another, Bull rush, Drink a potion, Feint, Overrun and Read a scroll. Refer to Attack Basics below.

Move

A move action lets you move your speed in a round or perform an equivalent action that takes a similar amount of time. Equivalent actions include climbing, drawing or loading a weapon, opening a door, and picking up an item. Refer to Move Basics below.

Full-Round Actions

A full-round action consumes all of your effort in a round. Attacking more than once (if you are of sufficient level to do so), Charge, Load a heavy or repeating crossbow, Light a torch, Run, or Withdraw are all considered full-round actions.

Free Actions

Free actions don’t take any time at all, though there may be limits to the number of free actions you can perform in a turn. Free actions include dropping an Item and speaking.

Five foot step

In any round when you don’t perform any other kind of movement, you can take a 5-foot step before, during, or after your other actions in the round.

—-Move Basics—-

Speed

Your speed tells you how far you can move in a round and still do something, such as attack or cast a spell.

Humans, Elves, Half-elves, and Half-orcs

Speed wearing no armor or light armor: 30 ft. (6 spaces)
Speed wearing medium or heavy armor: 20 ft.(4 spaces)

Gnomes and Halflings

Speed wearing no armor or light armor: 20 ft. (4 spaces)
Speed wearing medium or heavy armor: 15 ft. (3 spaces)

Dwarves

Speed: 20 ft. (4 spaces)
(Dwarves have no speed penalty for wearing armor.)

Moving

Tactical Movement
We use a battle grid to help keep track of where everybody is during combat. It is divided into 1 inch squares. One space on the grid represents 5 feet. Your character can move up to his speed rating in spaces (20 feet = 4 spaces) each round. Count every second square moved diagonally as 2 spaces. He may move through, but not stop in, a space occupied by a friend. Either before or after moving he may also attempt one standard action, usually an attack.
Double Move
If your character doesn’t do anything else in this round, he can move up to twice his speed. He is assumed to be on alert for potential threats, dodging arrows, avoiding blows from hand held weapons, and the like.
Charge
He can move up to twice his speed in a straight line up to an opponent and attack him. You get to add a +2 bonus to your attack roll because of the charge. This will be all that your character can do in this round so it is called a “full round action”.
Run
If your character doesn’t do anything else in this round, he can move up to 4 x his normal speed (or 3 x if wearing heavy armor). He is moving as fast as he can so he is not taking the time to avoid being hit from attacks the way he is if you just take a double move. Because of this, he looses his dexterity bonus (if any) to his armor class for the entire round.
Other Move Actions
There are rules for other forms of movement during a fight such as moving while balancing, moving silently, moving while attempting to hide, tumbling, climbing, swimming or crawling. Your character normally can’t use his full speed while moving in any of these ways and there may be other penalties as well.

—-Attack Basics—-

Making an Attack Roll:

Roll 1d20 and add the bonus listed for the weapon your character is using. If the result is equal to or greater than his opponent’s armor class, he hits. Then you can roll damage.

Making a Damage Roll:

Roll the type of die indicated for the weapon used and add its bonus (if any). Damage reduces your opponent’s hit points.

Critical Hits:

If you make your attack roll and it comes up 20 before any bonuses are added, this is called a “natural” 20. A natural 20 is always a hit. In most cases it is also a potentially critical hit. You then roll a second time and if the results of the second roll is also a hit then it is a critical hit. You then roll the damage twice. Whenever a you roll a natural 20 to hit and a natural 20 to confirm the critical, the resultant hit does maximum critical damage.

Some weapons will threaten critical damage on a natural 18 or 19. And with some weapons a critical hit may do 3 or 4 times normal damage.

Full Attack

Attacking more than once (if you are of sufficient level to do so) consumes all of your effort in a round. You must make the attacks in order from highest bonus to lowest. You can take no move actions or other actions this round except for free actions (such as speaking) and taking a 5 foot step. You can take a 5 foot step at any point during your round – before, during or after your attacks. All of the attacks don’t have to be against a single opponent. Melee attacks can be against anyone within reach or who comes within reach as a result of your 5 foot step.