Here is the DSM-IV criteria. You have to fit 5 of 9 symptoms, at least, to be diagnosed :

• Make frantic efforts to avoid real or imagined abandonment.
• Have a pattern of difficult relationships caused by alternating between extremes of intense admiration and hatred of others.
• Have an unstable self-image or be unsure of his or her own identity.
• Act impulsively in ways that are self-damaging, such as extravagant spending, frequent and unprotected sex with many partners, substance abuse, binge eating, or reckless driving.
• Have recurring suicidal thoughts, make repeated suicide attempts, or cause self-injury through mutilation, such as cutting or burning himself or herself.
• Have frequent emotional overreactions or intense mood swings, including feeling depressed, irritable, or anxious. These mood swings usually only last a few hours at a time. In rare cases, they may last a day or two.
• Have long-term feelings of emptiness.
• Have inappropriate, fierce anger or problems controlling anger. The person may often display temper tantrums or get into physical fights.
• Have temporary episodes of feeling suspicious of others without reason (paranoia) or losing a sense of reality.

Here is the diagnostic interview for borderlines, revised (found here) :

• Affect
• chronic/major depression
• helplessness
• hopelessness 
• worthlessness
• guilt
• anger (including frequent expressions of anger)
• anxiety
• loneliness
• boredom
• emptiness
• Cognition
• odd thinking
• unusual perceptions
• nondelusional paranoia
• quasipsychosis
• Impulse action patterns
• substance abuse/dependence
• sexual deviance
• manipulative suicide gestures
•  other impulsive behaviors
• Interpersonal relationships
• intolerance of aloneness
• abandonment, engulfment, annihilation fears
• counterdependency 
• stormy relationships
• manipulativeness
• dependency
• devaluation
• masochism/sadism
• demandingness
• entitlement 

Here are some attributes that people suffering from BPD can have, as found on this website :

• People with BPD are often bright, witty, funny, life of the party.
• They may have problems with object constancy. When a person leaves (even temporarily), they may have a problem recreating or remembering feelings of love that were present between themselves and the other. Often, BPD patients want to keep something belonging to the loved one around during separations.
• They frequently have difficulty tolerating aloneness, even for short periods of time.
• Their lives may be a chaotic landscape of job losses, interrupted educational pursuits, broken engagements, hospitalizations.
• Many have a background of childhood physical, sexual, or emotional abuse or physical/emotional neglect.

Traits involving relationships, as found on this website :

• Splitting: the self and others are viewed as “all good” or “all bad.” Someone with Borderline Personality Disorder said, “One day I would think my doctor was the best and I loved her, but if she challenged me in any way I hated her. There was no middle ground as in like. In my world, people were either the best or the worst. I couldn’t understand the concept of middle ground.”
   
• Alternating clinging and distancing behaviors (I Hate You, Don’t Leave Me). Sometimes you want to be close to someone. But when you get close it feels TOO close and you feel like you have to get some space. This happens often.
   
• Great difficulty trusting people and themselves. Early trust may have been shattered by people who were close to you.
• Sensitivity to criticism or rejection.
   
• Feeling of “needing” someone else to survive
   
• Heavy need for affection and reassurance