Shield bonus is a bonus to Armor Class granted by a shield or by a spell or magic effect that mimics a shield.
As with other sources of AC, shield bonuses granted by persistent feats (as compared to others that have spell-like abilities) are considered base bonuses and therefore stack with each other as well as the highest source of one other enhancement bonus (such as the sorcerer spell Shield). This has the effect of increasing shield bonus' overall addition to Armor Class. Improved Two-weapon defense is an example of a feat that grants a persistent shield bonus which stacks with the wizard spell shield.
Otherwise, shield bonuses stack with all other bonuses to AC except other shield bonuses.
Another example: a magic shield typically grants an enhancement bonus to the shield's shield bonus, which has the effect of increasing the shield's overall bonus to AC. The enhancement bonus granted by the magical aspect of the shield will not stack with other enhancement bonuses, but does stack with the bonus of the shield itself, as that is a base property. A shield bonus granted by a spell or magic item typically takes the form of an invisible, tangible field of force that protects the recipient.
A shield bonus doesn't apply against touch attacks.