Sexual Harassment, in short, is (for example) spanking or whistling at your co-worker every time you pass her at work or catcalling girls (e.g. an indian girl with a big ass: "her ass is bigger than the Taj mahal").
Rape is basically forcing your dick into an unwilling, unconscious, or unresponsive girl (or guy, for that matter.). It gets a bit more complicated than that at times, but generally the idea.