Simulate user input in a third-party application

Not sure where to start with this ...

I am thinking of creating a small small application to automate text rewriting. I would like to be able to identify pieces of text and enter them into other applications whenever I want. What would be the best way to approach this problem? I came across UI Automation as well as SendKeys. Any suggestions?

I don’t mind if I need to “specify” the input controls for the application (for example, as a screenshot tool for my native computer, it will ask you to click on the window you want to capture). For example, I could select a piece of text in this application, then click on another application text input field, and this will somehow allow the first application to get a handle to the input control to simulate input.

In addition, is there any way to programmatically “present” that form of control that would be completed? I come from the background of web design, so many of this native application are new to me.

Sorry if this is a vague question, I just don't know where to start from this ...