Life is you, me, and every living thing on our planet, but defining it nearly impossible. We study life here on Earth and look for it on distant worlds, so how can we not know what it is? The real problem is we only have one example of life: our planet.
All life on Earth is based on the same things. Carbon makes up the organic molecules; think of these as the Lego set of life. DNA is the code that allows for replication and therefore evolution. Water is the liquid that allows the organics to easily form long chains. All of these things have come together to make life here on Earth. What we don't know is if life could be based on other things, for example, liquid methane instead of liquid water.
Imagine that you had to define what a dog is, with only a St. Bernard as your example. A St. Bernard has four legs, slobbers all over, is very resilient to the cold, has a long snout, and is usually very friendly. By making a definition of all dogs by one breed, your definition becomes biased towards that breed. Some things like having four legs will apply to all dogs, but when you try to pin down specific attributes your definition breaks down fast.
So, what parts of our definition of life apply across the board, and when are we being too specific? We don't know. This is maybe one of the most important questions in science, because it directly relates to how we look for other forms of life in the universe. Could life be based on something other than carbon, maybe sulfur or silicon? Is there a code other than DNA that life on a distant world has utilized? The universe has always found ways of surprising us wherever we look. I see no reason why the search for life would be any different.