What is the defrance between Public companies & Privet companies?

Public companies raise money by issuing stocks and bonds to investors, who can trade the securities with other investors on the secondary market. Examples of public companies include Starbucks, Facebook, and McDonald's. 

This topic may related to SIE exam.

Private companies, on the other hand, are often owned by just one family or a small group of founders and investors. Their securities do not trade on a secondary market. Examples of private companies include The Chicago White Sox, Five Guys, and Koch Brothers.
Accessing the public markets provides a large amount of capital for the issuer. But, going forward, a public company must provide full disclosure of material facts to their investors, the securities regulators, and the public.

That is why many companies stay private. It is easier to run a business without having to disclose all material information, or worrying about liability to investors for failure to properly disclose something that leads to losses on their stocks or bonds.

If a private company goes public, their first offering of common stock is an initial public offering or IPO. In an IPO, a company sells a percentage of ownership to investors, using the proceeds to expand or accomplish other stated goals.

When the issuer receives the proceeds of the offer that is a primary offering or the primary market.

If early shareholders sell their shares to the public, we call that a secondary offering.

Many IPOs involve both the issuing company and the early investors selling shares to the public for the first time. This is known as a combined offering.

Facebook's IPO was a combined offering. According to the final prospectus, the company received approximately $6.7 billion while the early investors received just over $9 billion. Investors paid $38 for the IPO shares, which, as of this writing, trade for $178.30 on the secondary market.

Many IPO investments, however, do not work out for investors. Buying common stock is always risky. Buying an IPO is especially so.