SharePoint Online

This comes in two offerings:

Standard Offering	Dedicated Offering
Multiple customers share a common infrastructure	Each customer has its own infrastructure
Customer priority: rapid deployment	Customer priority: heavy customization
No lower seat limit	For customers with more than 5000 seats
Cost efficiency optimized for customers with up to 20,000 users	Optimized for customers with 20,000+ users

The dedicated offering support custom code including SharePoint Packages

For developing on SharePoint Online Services, check this white paper which describes the customization (or extensibility) capabilities that are supported with the Standard offering of SharePoint Online-

Microsoft SharePoint Online Standard Developer Guide (http://www.microsoft.com/downloads/details.aspx?familyid=d007f35e-375c-4b11-bc40-bc9082bb224a&displaylang=en&tm)