They don't have similarities at all. They are just written and spelled in a similar way.
"черт" is a word that means "devil".
"черта" means "line". But its used more rarely for this. There is another word for a line: "Линия". It's used more frequently.
"черта" is used more for some idiomatic expressions, like "провести черту над чем-либо (like "put a line in a sand" in English)"
Another meaning for a word "черта" - "черта характера" - means some feature of a persons character (something about his temper, etc)
