An IPO is the Initial Public Offering a company makes when first becoming a publicly traded company ie.listed on a national exchange. The FPO or Follow on Public Offering is the public issue of shares for an already listed company.

IPO comes first to Follow-up Public Offering as an FPO can only be given if there is an initial public offering.
IPOs are more profitable than FPOs. A company makes an IPO for compiling money and an FPO for adding to the initial public offerings.
Initial Public Offering is the first sale whereas the Follow-up Public Offering is the second sale for expanding businesses.
IPOs are risky investments as an individual investor cannot predict what will happen to the initial trading in the coming days. But in the case of FPOs, the risk is lower as an investor already has an idea about the investment and future growth of the company.