need to understand a few things about caching

i have in my sidebar a few homemade “widgets” that show in a div the most recent 10 posts’ titles, the most recent 10 comments excerpts, the most recent 10 articles’ titles, etc. I also have a poll box and some other interesting stuff will come.

i have some lacks of info on caching. As i have read if i get some plug-in my pages will be saved in the cache and a next time the page will be requested the server will serve the cached page…unless a comment was added(right?).

1) Does this mean that the most recent 10 stuff is served as it was if there was no recent comment that forces wordpress to delete the cached querries? Or this is applyable for any change like a new post or a new article?

2)The other answer would be that every time a page loads wordpress reads the database for each query any widget might request. Can anyone explain in short my thoughts/questions?