I am new to Office 365. Can any one tell me whether custom webparts (sandboxed) cabe be built as an app to sharepoint online? is yes the how a webpart can be published as an app ?
I would recommend you have a look at this! it outlines everything that you would need to know! it should also be a very good starting point! the sandbox solutions can be found near the bottom what goes into detail. Yes it a big read but it answers all your questions
hope it helps :)
Office 365 for developers download for VS2012 (download and install)
Blog - someone who istalled the above to develp on VS
How to: Install the Visual Studio Tools for Office Runtime Redistributable
Office Development in Visual Studio
hope this helps :)
Since your looking for developing a webpart for Office 365, you can follow this tutorial, it shows you everything step by step
I would not name web part as application and I never bother about sanboxing since it is impossible to have non-sandboxed apps in Office 365.
The easiest way for a newbie to develop a web part is to publish any Infopath form which then appear in list of available Infopath web parts. The latter you can see/use by navigating to or create in web browser to any web part ASPX page in Sharepoint online (Office 365) site > click Add a web part link > Forms (under Categories) > Infopath Form Web Part > Add > Click here to open the tool pane link > in tool pane under List or Library: list box you will see the list of published Infopath forms
The easiest way to create and publish an Infopath form is to navigate in browser to any list in Sharepoint online site > click Customize Form on ribbon > in Opened Infopath Designer 2010 click Quick Publish button (on ribbon File > Info) ... after or without its editing
That's it, you have developed, published and used ("sandboxed") web part.