A lightworker, is a spiritual person who believes that they're incarnated on Earth in human form for the specific purpose written in their soul's divine blueprint. This purpose is to raise the vibration of the planet (see: The Law of Vibration) and spread light-based energies to the earthly dimensional plane (see: Dimensions, Universal Tree of Life; also Sefirot and 10 Dimensions of Reality in Kaballah). Lightworkers basically believe that they are here to share and spread the light that is currently in existence. source
Enlightenment is the common, shared goal of Lightworkers and Masters. When a person has a higher understanding of spiritual concepts, they are generally referred to as enlightened. However, in truth, being enlightened means holding a certain amount of Light. - gaia.com
Lightworkers are not associated with any specific religion, or earthly organisation. It's a different perception of reality (belief system), similar to Kaballah (an ancient spiritual wisdom) which explain that the universe is made of different dimensions of reality (see: string theory).
In other words, "light work", means light doing (spreading the light). So by doing "light work", they help humanity to ascend (also known as spiritual awakening) to higher vibrational planes of existence (similar as Jesus did or other ascended masters).
Lightworker term could be new, however ancient spiritual texts such as Bible is using "light" in many verses. For example, about having light within ("your whole body also is full of light" - Luke 11:34-35; "you are the light of the world" - (Matthew 5:14), so it's a similar concept (about spreading the light), given the right interpretation. Basically, lightworkers believe that they're made of light (a manifestation pattern of electromagnetic energy, an intelligent energy), as everything else in the universe (see: holographic universe theory, consciousness as a state of matter).
It's worth to note, that there is also a similar term: a starseed.
The differences between these two types of individuals has to do with the soul's origin, their core objective, and the foundations of their expertise.
Both are on the planet to uplift the energies of light.
Source: "Lightworker" vs. "Starseed"
Read more: What is lightworker?