It is the sum of all annual commercial/capital/investment gains (after having subtracted your loss and expenses).

Note: If you still follow Sayyid al-Khoie, then you may add up the profit and losses from all your business ventures. However if you follow Sayyid al-Seestani then you must calculate your gain from separate businesses/investments separately (i.e. you cannot subtract the loss of one business from another to reduce your gain).

Usually different accounts, such as checking, saving and CDs gain some interest. As long as this interest is not spent, then a khums of it must be paid.

If real estate (for living of business) or some commodities (furniture and etc) are part of your needs and are used by you, then any gain in their value is not subject to khums.

But if the Real Estate (or other property) is in excess of what you need and/or use, then you must pay the khums of its gain [When you sell it???] (Assuming that said property was bought with money that khums on it had been paid already).

Scholars say that a small gift is not subject to khums, even if it is not used. However, if the gift is valuable but it is not used/needed then a khums of it must be paid.