I am trying to develop a plugin and what I want to do in this plugin is to be able to access the contents table that is already defined in the database and get and update entries in this table. What would be the best practice to do this? I know records can create new tables but how can I access or manipulate an already created table that is not part of my plugin but is created in the database by default?
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:
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...
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.