The perception ability is best defined as a character's eyesight and hearing.  It is more than just that however.  It also represents the characters "attentiveness, visual and aural memory, and interest in his surroundings".  Expanded its meaning to include the degree of Extra-Sensory Perception.  This does not mean that characters with high scores will have ESP, or any other psionic-like abilities.  It does mean that this ability modifies proficiencies that are related to the sixth-sense (see below), and should be used for ability checks in cases involving extra-sensory perception.
          Perception will come into play a lot in a campaign.  It should be used as the ability check for such things as:  detecting someone who is following the characters, noticing the stranger in the corner of the tavern is looking at you, seeing a small hole in the wall of a dungeon room, etc.  The possibilities are endless, and it is much more comfortable using this ability for those checks than trying to justify a Wisdom, Intelligence, or Dexterity check.
          9-11 is normal human 20/20 vision.  7 or below means sight is impaired, but a score of 5 or above can be corrected using spectacles (adds 4 to score, but will not raise score above 11). Information from an article by Necromncr@aol.com [Schwartz Lair]

Hearing					Hear Noise *
Roll 1d6				Race		Percentage	1d20
1      hearing is 1d4 worse			Dwarf/Half-Elf/Human	15%		   3
2-5   hearing is the same			Elf/Halfling		20%		   4
6      hearing is 1d4 better.			Gnome			25%		   5
			* Add Skill Adjustment from Perception to the base chance

Ability	Reaction	Proficiency	Skill	Medium/Long Range
Score	Adjustment	Adjustment	Adjustment	Missile Adjustment
1	-4		-5		-25%		-3/-4
2	-3		-4		-20%		-2/-3
3-4	-2		-3		-15%		-1/-2
5	-1		-2		-10%		-1/-1
6-7	-1		-1		-5%		 0/-1
8-14	0		0		0		 0/0
15-16	+1		+1		+5%		+1/0
17	+2		+2		+10%		+1/+1
18	+3		+3		+15%		+2/+1
19-20	+4		+4		+20%		+2/+2
21+	+5		+5		+25%		+2/+3

Reaction adjustment:  bonus to surprise roll
	Normal Surprise Roll [1, 2, 3 on 1d10]
Proficiency Adjustment:  the bonus applies to NWP that are somewhat related to sight or hearing.
	Also to Detect Doors & ALL Dwarf/Gnome/Halfling Underground Detection Abilities.
Skill adjustment:  The percentage applies to skills like Hear Noise or Find Traps and also to the [% to Notice] Pick Pockets.

Class Minimums
Ranger	Pcp:  15
Druid, Thief/Rogue	Pcp:  12
Bard	Pcp:   9

Affected Skills
Ranger	Tracking
Thief	Hear Noise, Find Traps
PO:S&P	Detect Illusion, Detect Magic, Escape

NWP with Perception Bonuses

General:  Cartography, Eavesdropping, Enemy Reading, Fishing, Fungi Recognition, Gaming, Grooming (Animal), Grooming (Humanoid), Heraldry, Hiding, Modern Languages, Navigation, Orienteering, Sign Language, Signaling, Sound Imitation, Whistling/Humming.  Dwarf Runes [Dw]. 

Priest:  Astrology, Astronomy, Botany, Calligraphy, Geology, Investigation, Ancient Languages, Reading/Writing, Somatic Concealment, and Spellcraft.

Psionicist:  Body Language, Camouflage, Reading/Writing, and Sleight of Hand.

Rogue:  Animal Noise, Appraising, Camouflage, Courtly Intrigue, Cryptography, Disguise, Foraging, Forgery, Information Gathering, Sleight of Hand, Trail Marking, Trail Signs, Tailing, Trailing, Voice Mimicry.

Warrior:  Camouflage, Foraging, Hunting, Tracking, Trail Marking, Trail Signs

Wizard:  Astrology, Astronomy, Botany, Calligraphy, Geology, Ancient Languages, Prestidigitation, Reading/Writing, Somatic Concealment, Spellcraft, and Tactics of Magic

		Perception Proficiencies
Class		Start		#Level
Priest		2		3
Rogue		2		4
Warrior		2		3
Wizard		2		3

GENERAL
Proficiency	Slots	Ability	Modifier
Alertness (NWP/WP)	1	Per	+1
Danger Sense	2	Per	-1
Direction Sense	1	Per	+1
Distance Sense	1	Per	0
Observation	1	Per	0
Sixth Sense	2	Per	-2
Sound Analysis	1	Per	0
Time Sense	1	Per	0
Trouble Sense (WP)	1	Per	0
Weather Knowledge	1	Per	-1
               Weather Sense			

PRIEST
Magical Awareness	2	Per	-1
Planar Sense	1 	Per	-1

PSIONICIST
Psionic Detection	1	Per	-2
Psionic Lore	2	Per	+1

ROGUE
Awareness	2	Per	0
Dark Sense	1	Per	+1
Night Vision	1	Per	-2
Reading Lips	2	Per	-2

WARRIOR
Awareness	2	Per	0
Dark Sense	1	Per	+1

WIZARD
Dowsing	2	Per	-1
	1	Per	-3 [S&M]
Magical Awareness	2	Per	-1
Planar Sense	1 	Per	-1