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An RSU is more like a stock grant than a purchase option -- the company provides shares of stock to its employees according to a vesting schedule. The fair market value of the shares at the time of vesting is considered income, and there is no cost to the employee (other than income taxes).

I don't know if this is a US-only arrangement, or if it's used in other countries as well.

The other repliers added more info, sorry.

You are correct about share options, on which your gain is only the price difference for which you pay CGT (or not). But the liquid tech companies (Google, FB, Twitter) give out direct stock which is taxed on their Fair Market Value at the time of vesting. RSUs are taxed the same way both in UK and US.

RSU = restricted stock unit whether you're in the UK or the USA.

Never heard of RSU's in the UK context - UK taxation depends on if its an approved scheme or not.