Invalid prayer

As-Salam `alaykum! Would you tell me the things that invalidate my salah (ritual Prayer)? All the time I am confronted by people who talk about so many things saying that they invalidate my Prayer. What are the real things that render my Prayer invalid?

Respected scholars, as-salamu `alaykum. I have a question that has been bothering me for years.  I believe I suffer from anxiety that manifests in extreme sweating on my head.  During salah (ritual Prayer), usually when in congregation, I cannot control the sweating and it literally drips off my head and onto my face.  Is it permissible for me to carry a handkerchief with me during salah to wipe the sweat? Jazakum Allahu Khayran.