Shares
A share is the single smallest denomination of a company's stock. So if you're divvying up stock and referring to specific characteristics, the proper word to use is shares.

Technically speaking, shares represent units of stock.
Common and preferred refer to different classes of a company's stock. They carry different rights and privileges, and trade at different prices. Common shareholders are allowed to vote on company referenda and personnel, for example. Preferred shareholders do not possess voting rights, but on the other hand, they have priority in getting repaid if the company goes bankrupt. Both types of shares may pay dividends, but those in the preferred class are guaranteed to be paid first if a dividend is declared.

Common and preferred are the two main forms of stock shares; however, it is also possible for companies to customize different classes of stock to fit the needs of their investors. The different classes of shares, often designated simply as "A," "B," and so on, are given different voting rights. For example, one class of shares would be held by a select group who are given perhaps five votes per share, while a second class would be issued to the majority of investors who are given just one vote per share.