The deployment models for Apps in SharePoint are
- SharePoint-hosted - App and all resources will be hosted in your SharePoint farm. Used for light weight smaller applications using HTML,Java Script and Client object model. Here you do not need to write server-side code. scope is site collection level.
- Autohosted - App is hosted in cloud. Used for light weight applications but the code is deployed at windows azure using cloud. scope is at site level.
- Provider-hosted - App and all resources are hosted on separate server. Code does not exists on SharePoint but will be in other domain which can be windows azure, IIS or even PHP app from your domain server.
No matter where you host your app, it should target to support and consumable in SharePoint Apps.