It's the same process, but STOs issue tokens on a blockchain while IPOs issue share certificates on traditional markets. Although both are regulated offerings, IPOs are only used in private companies that want to go public. Through the IPO process, they raise funds by issuing shares to accredited investors.
With STOs, tokens that represent a share of an underlying asset are issued on the blockchain to accredited investors. These can be shares of a company but, because of tokenization, can really be of any asset that is expected to turn a profit, including a share in the ownership of a property, fine art, investment funds, etc.
STOs are also more cost-effective than IPOs. With IPOs, the companies would typically pay high brokerage and investment banking fees to get access to a deeper investor base. STOs would still need to pay lawyers and advisors, but they offer more direct access to the investment market and, therefore, typically won't have to pay large fees to investment banks or brokerages. The post-offering administration for STOs is also less cumbersome and cheaper than with traditional IPOs.