Mental illnesses come in many forms and with various levels of impact.
Accordingly there are many different diagnoses determined by professionals as detailed in DSM-V (Diagnostic Statistical Manual version V).
This website will present only a broad overview of some major mental illnesses with limited discussion about these illnesses as I have seen and been involved with them. (There are many other websites and resources that can provide much more detailed information. As Always, I encourage you to seek professional medical advice from licenced practitioners.)
Some symptoms are common across these illnesses, and I have also found that symptoms can sometimes change over the course of an illness both in the specific symptoms presented and in the severity of the symptoms.