Issued shares are shares which have been allocated by the company to its shareholders.
The issued shares are owned by the shareholder. They represent an ownership interest in the company. A company can issue new shares up to the amount of authorized share capital. The number of issued shares may be less than, but cannot be more than, the number of authorized shares.
If you all of the issued shares in a company, you own 100% of the company. If there are 100 issued shares, you still own 100% of the company whether the company has 100 authorized shares or 1,000,000 authorized shares.
Issued shares can be sold by the shareholder or, if the company allows it. Shares which may be issued to an option holder in the future are not issued shares. The shares are not issued shares until the option holder actually exercises the option.
The number of issued shares can fluctuate. It will increase if the company issues additional shares, for example in a future financing round. It can also decrease if the company chooses to buy back some of the issued shares.