We know how list oriented Sharepoint is. Most components that you interact with MOSS/WSS are list based – doc libraries, form libraries, tasks, links, etc, etc. You can setup alerts on them, get feeds on them or download offline copies of them and so on. I love Sharepoint lists.
The processes you build with K2 can be surfaced as sharepoint workflows. They can be kicked off automatically or manually from lists. The point is you can build your own lists and utilise the form interface it provides for creating/editing the list as the initiation UI for processes. You can also use the edit interface as the interaction interface.
If you bind another workflow to handle this edit action, i.e kick that off when a change is made to list entries, you can easily use the list meta data to correlate with the task that is sitting on the user and finish the task from within the workflow.
K2 has the power to automate many aspects of MOSS including permissions and your process can then change the permissions of the list item to give access to the task user and bingo now you have secured the list item for editing.
So you can use Sharepoint UI – the list editing form interface – to power your K2 business processes.
Of course when you find this interface is not enough for the things you perhaps want to do in your UI, you can use Infopath. K2 integrates with Infopath. You can create an infopath based business process and deploy it from within the K2 designers and deploy without writing a single line of code or XML.
If you want to use web forms and windows forms then you can use the APIs. Samples are provided and they are very easy to do.