Snap Inc. lifted the veil on its highly anticipated initial public offering, revealing a business that is growing at a torrid clip but that also faces challenges keeping users engaged, attracting new ones—and justifying a valuation that could reach $25 billion.

The parent of the popular Snapchat disappearing-message app, which had previously filed IPO papers confidentially with the Securities and Exchange Commission, made its so-called S-1 public Thursday, ahead of a share sale that could take place as early as the first...