Guardians are a type of creature who usually fight for truth and justice. They usually use Light Element based abilities. The most common power they use is Grace, allowing them to protect themselves from each other's attacks whenever they fight each other.
The term Guradians was just recently adopted by Flora. It actualy changed various times, it was changed due to story changes or religious reasons.
The first term it was known as were "Deities". This was changed due to Religious reasons. At this time Nightmares also were under the term "Demons". The second term was short lived, Guardians were known as "Forces of Nature" and Nightmares respectively were "Forces of Mechanics" or creatures who aren't organic. This was changed, since the series of GALAXY changed soon after this time.
There are a bunch of Guardians for GALAXY, these are the current confirmed ones: