If the owner takes any assets out of the business, the business owes the owner that much less. It is good for the owner to know how much they have taken out over a period of time so this amount is kept separate for a period of time also. The amounts in all the other owner's equity accounts are eventually recorded in the Capital account. Any monies the owner invests in the business are entered directly into this account. The amount in this account is eventually recorded in the Capital account and decreases the Capital account.
Revenue is what a business earns for what it is in business to do.
For example, what a dry cleaning business charges for cleaning a suit is revenue. The amounts in revenue accounts are eventually put in the Capital account and these amounts increase the Capital account. Expenses are the cost of generating revenues. In other words, expenses are the cost of doing business. The amounts in expense accounts are eventually put in the Capital account and these amounts decrease the Capital account.