When considering stand-alone risk, the return distribution of a less risky investment is more peaked (“tighter”) than that of a riskier investment

When considering stand-alone risk, the return distribution of a less risky investment is more peaked (“tighter”) than that of a riskier investment. What shape would the return distribution have for an investment with (a) completely certain returns and (b) completely uncertain returns? What are the two types of portfolio risk? How is each type defined? How is each type measured?