A public issue is when a company sells its shares or securities to the public to raise money. This can include an Initial Public Offering (IPO), where a company first offers its shares, a Follow-on Public Offering (FPO), where an existing company sells more shares, or an Offer for Sale, where current shareholders sell their shares. The process is regulated by SEBI and involves filing a detailed prospectus. The company and market conditions set the share price, and intermediaries like investment banks help manage the issue.