A FreeBet is an amount that can be used to place a sports bet. If your bet wins, the net winnings (winnings minus stake) will be credited as cash into your account balance.
Free Spins are spins to use when playing Casino. They can be available immediately or it might be required to make a minimum deposit to activate them. Also, they might work only on specific slots. Specific requirements will be visible after clicking "More Info" in the Offers" section of your account.