If someone is sad or anxious they might be in a depressed mood
Yes its a new age problem thats growing bigger every day, are you anxious sometimes?
No not really. I've known people with Borderline personality disorder though. 
What is that? i havent heard of it
It's a long-term pattern of unstable behaviors, loss of identity, and promiscuity 
Omg its really bad, now i know, what can you tell me about depression, i think i feel depressed sometimes
Depression (mood)
Depression is a state of low mood and aversion to activity. It can affect a person's thoughts, behavior, motivation, feelings, and sense of well-being. It may feature sadness, difficulty in thinking and concentration and a significant increase/decrease in appetite and time spent sleeping, and people experiencing depression may have feelings of dejection, hopelessness and, sometimes, suicidal thoughts. It can either be short term or long term. Depressed mood is a symptom of some mood disorders such as
the depressed mood caused by bipolar disorder, but a positive event would potentially lift the depressed mood of someone with BPD. Similarly, an undesirable event would not dampen the euphoria caused by bipolar disorder, but an undesirable event would dampen the euphoria of someone with borderline personality disorder.
Third, when people with BPD experience euphoria, it is usually without the racing thoughts and decreased need for sleep that are typical of hypomania, though a later 2013 study of data collected in 2004 found that borderline personality disorder diagnosis and symptoms were