What is the different between realised & Unrealised (Gain or Losses) in Securities?

Gain and loss are said to be "realized" when a stock (or other investment) that you own is actually sold. Unrealized gains and losses are also commonly known as "paper" profits or losses. So all gains or losses are unrealized to you sell securities

An unrealized loss occurs when a stock decreases after an investor buys it, but he or she has yet to sell it. If a large loss remains unrealized, the investor is probably hoping the stock's fortunes will turn around and the stock's worth will increase past the price at which it was purchased. If the stock rose back above the original price, then the investor would have an unrealized gain for the time he or she still holds onto the stock.

An unrealized loss occurs when a stock decreases after an investor buys it

and when it sold the realized gain or losses appears 

another deference is the presentation income statement or OCI