Ethical theories attempt to explain, defend, systemise and justify varying concepts of 'right' and 'wrong' behaviour.
The concept attempts to analyse the values which underpin social decisions that may be considered 'good' or 'bad', and uses these valuations to determine why certain things happen, and why certain choices are made.
It is worthwhile to note that all of these values pertain to moral relativism, which means that morality is applied relatively and subjectively, hence the need for ethical theories to determine these nuances.
This system specifically relates to human conduct and social tendencies, including the actual action itself, intertwined with the ends of said actions (a means to an end). It is essentially an analysis of values.
A famous study is the case of Baby Doe, who was born with Down Syndrome, plus many other medical difficulties that would make caring for the child, and it's life subsequent to surgery extremely difficult. The parents chose not to undergo the surgery, shortly after which, the baby died. This resulted in a surge of controversy.
Ethical theories attempt to outline the 'good' or 'bad', and the 'right' or 'wrong' in these very situations.
The concept of 'Ethics' falls into two separate categories. 1. Theoretical ethics - a systematic attempt to justify and understand moral concepts and principle theories. 2. Applied ethics - concerned with the controversial application of morality, e.g euthanasia, capital punishment and abortion.
I hope this answers your question fully!