"explain the difference between risky and riskless arbitrage."

Risk arbitrage also called merger arbitrage trading; it refers to an event mediated hypothetical trading method. It tries to produce profits by taking a long position in the stock of a target company, and optionally merging it with a brief position in stock of an attaining company to produce a verge.

Riskless arbitrage includes taking merit of interest rate differentials by involving in a spot transaction today to sell/buy foreign currency, and at the same time involving in a purchase/sale of foreign currency for a particular time in the future.