Any time you multiclass, you gain nothing but the features of your new class. Your previous classes don't increase. This is the same regardless of which new class you enter.
Instead of gaining the abilities granted by the next level in your character’s current class, he can instead gain the 1st-level abilities of a new class, adding all of those abilities to his existing ones. This is known as “multiclassing.”
For example, let’s say a 5th-level fighter decides to dabble in the arcane arts, and adds one level of wizard when he advances to 6th level. Such a character would have the powers and abilities of both a 5th-level fighter and a 1st-level wizard, but would still be considered a 6th-level character.
– Pathfinder Roleplaying Game Core Rulebook, p. 30
Sadly, the Core Rulebook does nothing to explain how prestige classes work, just presenting them as extra classes that have prerequisites to enter, but the rules above still apply.
There are, occasionally, some classes that increase existing class features. These classes will tell you when and how it happens. Most commonly, spellcasting prestige classes will effectively increase your position on the spells table of a spellcasting class you possess (and not, for instance, bloodlines, or school powers and arcane bonds).