What Are Dream Signs?
Dream Signs are clues within a dream which can trigger the realization that you are dreaming and become lucid. Because dreams tend to deal with specific and repeating matters, we can learn about dream-themes in general or our own specific dreams and through familiarization more easily recognize dreams while still in them.
The dream-generating-system we were all born with is an evolutionary adaptation which allows humans to practice perceiving and dealing with threatening situations. For other mammals, dreams allow each species to practice whatever things which best prepare that organism for survival. For instance, cats dream about hunting small animals; gazelle probably dream about running from lions, etc. Humans dream about being chased by men from other tribes or large cats, or falling (remember, we were tree-dwellers once), social issues, sexual relations, etc.
The basic formula for human dreams is: trying again and again to deal with some threat that won't stop. Yes, there are exceptions, but perseverance in the face of a threat is the typicality. When we are young, dreams are filled with animal pursuits and being chased by men, and as we get older modern culture and its new threats (i.e., being late for something, cell phones or computers not working properly) dominate the dream world. If you have no stress in your life (who are you, anyway?), then you either do not have vivid dreams or your dreams may seem mundane and boring, maybe even fun.
So, that covers what dream material is like, generally. To get a better idea of what your own particular dreams deal with you will need to keep a dream journal, even if only for a few weeks at a time. If you have poor dream recall (and you are not taking anti-depressant drugs which diminish REM-sleep), you will be amazed at how much better your dream recall can get by simply trying to pay attention to your nightly episodes and writing or typing them down (hint: Do not move a muscle when you first wake up until you have mentally gone over what you have dreamed). What you should notice are patterns, especially dreams that close together in time. Maybe you have a big project coming up at work and this triggers dreams of being back in school taking a test and the questions don't make any sense. Perhaps you are in a relationship and your partner cheats on you in a dream. Whatever material is in your dreams, the idea is to become aware of what to expect, and when these "dream signs" show up again you will better prepared to recognize "Oh, I must be dreaming!" When this occurs, test the reality of your situation using the method of your choice (text-stability is a common beginner test), and if the test comes up positive for lucidity, stabilize by spinning or using a mantra (e.g., "More lucidity now!") and be on your way to adventure.
Noticing dream signs is obviously tricky without practice, because the parts of your brain responsible for critical thinking are not highly activated in regular dreams. This is why we accept dreams for normal reality during the majority of our life. Recognizing dream-signs is a skill and requires practice, but this is one of the most powerful tools you should have in your kit when aiming for lucid dream induction.