The following are changes to the Combat system as presented in Chapter 8 of Core Rulebook I:
Touch AC & Shields
In most cases, characters will be allowed to add any Shield bonus granted to them to their Touch AC. (It is anticipated that in 3.5 Shield Bonus will be a recognized bonus type instead of listing shields as a special Armor Bonus that stacks with armor.)
Note that there may be some situations where you do not get your shield bonus included into your AC. Shocking Grasp vs a metal shield is a good example of an exception. Attacks of incorporeal creatures, which ignore material armor (unless it is made of force) still also ignore shields.
Missile Fire into Mêlée
Unless you have a Feat which allows accuracy when firing into mêlée, you will have a chance to hit someone other than your intended target.
If all characters and creatures engaged in mêlée are of the same size, then you have an equal chance to hit any of them.
If they are different sizes, it is twice as likely to hit the larger size category.
For example, if 2 small, 2 medium and 3 large sized beings are in a mêlée into which you fire an arrow, you would first determine who you might strike.
On a d10:
1 - Small creature #1
2 - Small creature #2
3-4 - Medium creature #1
5-6 - Medium creature #2
7-10 - Large creature
The roll for who you strike and your to-hit roll are considered to be simultaneous rolls.
DeathA character dies when they reach negative the character's Constitution + character level. For example a 10th level character with a 15 CON would be 'dying' between -1 and -24 and be 'dead' at -25.
Attacks of Opportunity
Read the Sage Advice article which explains them more clearly.
Turn Undead
This works as per the ability described in the PH with the following change. Free-willed undead can choose not to flee but instead will be held at bay. As long as the character maintains concentration, undead will not be able to approach any closer than 10' to the character (+/- one foot per level difference between their level and the character's level, to a minimum of 1') and they can not take any action in regard to the character and her allies other than engaging them in conversation. Likewise if the character or any of her allies takes any action other than conversation with the Free-willed undead, the effect of the turning is broken. If any of the cleric's allies approaches closer to the undead than the 'at bay' minimum distance, they are not considered to be protected by the cleric's power any more.