people that are self-conscious are more aware of their looks. 
I don't know a lot about that can you explain?
It is more of a preoccupation with oneself, as opposed to philosophical self-awareness
Like being self absorbed or just fixated on oneself?
No it is associated with being shy and embarrassed. They fee like they are not good enough.
Oh so like a insecure complex?
Yes and it results in lack of pride and low self-esteem.
Is it a mental illness?
Self-consciousness
Self-consciousness is a heightened sense of self-awareness. It is a preoccupation with oneself, as opposed to the philosophical state of self-awareness, which is the awareness that one exists as an individual being, though the two terms are commonly used interchangeably or synonymously. An unpleasant feeling of self-consciousness may occur when one realizes that one is being watched or observed, the feeling that "everyone is looking" at oneself. Some people are habitually more self-conscious than others. Unpleasant
feelings of self-consciousness are sometimes associated with shyness or paranoia.
Impairment.
When feeling self-conscious, one becomes aware of even the smallest of one's own actions. Such awareness can impair one's ability to perform complex actions. 
Adolescence is believed to be a time of heightened self-consciousness. A person with a chronic tendency toward self-consciousness may be shy or introverted.
Psychology.
Unlike self-awareness, which in a philosophical context is being conscious of oneself as an individual