A capital gain is an increase in the value of an asset while a loss refers to the loss of value. Capital gains and losses can be realized or unrealized.
Realized gains and losses occur when you actually sell or dispose of the asset. Capital gains are taxed only when they are realized, while capital losses are deducted only when they are realized.
Unrealized gains and losses happen when an asset's value changes after it is purchased and hasn't yet been sold. Unrealized gains are not taxed by the IRS, just as unrealized losses can NOT be deducted from your taxes.