This comp will provide information regarding how Rounds work, Actions you can take during these Rounds and how Movement is measured.
In D&D 3.5, every Round is equal to 6 seconds. Anything a person could
reasonably do in 6 seconds, your character can do in 1 round. Rounds in Combat follow this pattern:
1. Each combatant starts out flat-footed. Being flat-footed is your Armor Class minus your Dexterity modifier. Once a combatant acts, he or she is no longer flat-footed.
2. The DM determines which characters are aware of their
opponents at the start of the battle. If some but not all of the
combatants are aware of their opponents, a surprise round
happens before regular rounds of combat begin.
3. Combatants who have not yet rolled initiative do so.
All combatants are now ready to begin their first regular
round of combat.
4. Combatants act in initiative order (highest to lowest).
5. When everyone has had a turn, the combatant with
the highest initiative acts again, and steps 4 and 5 repeat
until combat ends.
Surprise rounds occur when only some of the party are aware of trouble. If any are aware they get to act before the other party members, rolling initiative and making actions. If no one or everyone is surprised, no surprise round occurs. This is not the same as a "Sneak Attack".
Actual "Sneak Attacks" are a Rogue Class skill. Anyone can catch an enemy unawares and roll against their Flat-Footed AC. A Rogue's Sneak Attack still targets the same AC but they roll for extra damage. This extra damage is 1d6 at 1st level, and it increases by 1d6 every two rogue levels thereafter. A Level 5 Rogue would roll 3D6 when determining her Sneak Attack damage.
The big 3 action types you can do in a round are as follows:
Standard Action: A standard action allows you to do something. The most common type of standard action is an attack—a single melee or ranged attack. You could also cast a spell, use a magic item or special ability.
Move Action: A move action allows you to move your speed or perform an action that takes a similar amount of time like drawing/sheathing a weapon, standing back up or picking something up.
Full-Round Action: A full-round action consumes all your effort during a round. The only movement you can take during a full-round action is a 5-foot step before, during, or after the action. Attacking twice in one round due to a high Base Attack or special weapon is a Full-Round Action. So is flat out Withdrawing from a fight and running away.
THE 5-FOOT STEP
Every square on the grid map represents 5 feet. You can move 5 feet in any round when you don’t perform any other kind of movement. Taking this 5-foot step never provokes an attack of opportunity. You can’t take more than one 5-foot step in a round,and you can’t take a 5-foot step in the same round when you move any distance.
You can take a 5-foot step before, during, or after your other actions in the round. For example, you could draw a weapon (a move action), take a 5-foot step, and then attack (a standard action), or you could cast fireball (a standard action), take a 5-foot step through an open door, then close the door (a move action).
Difficult terrain and darkness can impair a 5-foot step.
LINE OF SIGHT
From your character's spot, draw a line from any of the 4 corners to your opponent's square. If even 1 line can reach an enemy's corner without so much as touching a wall or other obstacle, you have line of sight. This applies to ranged attacks and spells requiring line of sight.
Using the same "draw a line" rule from Line of Sight, if a line from your corner to the enemy's corner goes thru or touches a wall, they are considered to have "Cover" and gain +4 to their AC. As the examples above show, even other characters can give cover to both friend and foe alike, intentionally or not.
Flanking is the quickest way to gain an advantage if you have numbers on your side. When making a melee attack, you get a +2 flanking bonus to your attack roll if your opponent is threatened by a character or creature friendly to you on the opponent’s opposite border or opposite corner.
Thanks for reading! I apologize if this one was a little dry but hopefully I was able to clear up any confusion. As always feel free to leave questions and a thumb if you're feeling generous.