Using a plugin, how can I manipulate entry data?
You are correct in stating that Records typically define a new custom table for your plugin. However, don't forget that Craft already comes with many pre-existing Records.

Take a look inside this folder:

/craft/app/records


Although based on your question (and the ensuing comments thread), it looks like you may not need to tap directly into any Records at all. I'd recommend you peruse the built-in API to see what may help you get the job done.

Specifically, take a look at the EntriesService API... 


  http://buildwithcraft.com/docs/apis/entriesservice


I suspect that everything you need to do can be done using those methods. After all, they already allow you to get, modify, and delete entry data.

An important thing to note when dealing with the EntriesService API... You're not directly accessing Entry Records. Instead, you'll be dealing with Entry Models. This is greatly preferred, and you'll find that it's much easier to do what you want with a Model. (Behind the scenes, your model will tie into an Entry Record as needed.)



On a separate note, it looks like you'll need to take advantage of Events. You mentioned in a comment that you'd like an action to take place "when an entry is saved". To do that, you'll definitely want to take advantage of the entries.onSaveEntry event.



In summary, I don't think that you will need to create a custom Record, nor build your own custom Yii query.