This depends whether you're making an internal app for your organization and its employees, or if you want to distribute an app to anyone in the general public.
For Android distribution you can post the file publicly online & anyone, anywhere can download and install it if they allow their device to do so. This is not possible with iOS apps for security.
Your options for apps aimed at the general public are:
- The public App Store
- Test Flight w/user emails
-- (up to 10K users; 90 days per build; minimal internal testing of a few dozen users; external test builds must pass App Store review first)
- "Hockey App" which will soon become Microsoft's "App Center" allows for test builds and distribution as well.
- Collect and manage individual UDIDs for manual testing (the "old school approach", also limited # of devices per category).
If your app is for internal, organizational use by a restricted group of employees, you can publish to an internal "App Store" setup w/your Mobile Device Management system (Citrix SecureHub, XenMobile), or post the .ipa file to a company web server as detailed in this article. No Apple/App Store review is required.
This requires being a member of Apple's Enterprise Developer Program and signing your apps with the Enterprise Developer Certificate. This program is a separate, more expensve ($299/yr) cost/subscription designed for organizations to publish/distribute "in-house" apps solely for use by their employees.
Users who then install your app from the company web server must "Manually Trust an Enterprise Developer" as explained in Apple's docs on distributing in-house/ad-hoc apps.
EDIT:
As you mentioned, there's also the "B2B" route for, it seems, a regular non-enterprise developer, but this also requires that your end user be a business with a MDM setup or redemption codes purchased as part of a corporate "Volume Purchase Program." Still doesn't work to just post the .ipa online as you're hoping.
See this description & WWDC video if you're interested in this route.
Also, this question is essentially a duplicate of this earlier question...