Hi jezzze,
That's exactly it, you got it - Any goal you have set up in your marketing efforts would then therefore be recognized as a conversion. So e-mail sign up, get more info on a service, etc. can all be classified as such. The difference in the ROI and conversion tracking is when your campaign is set up for different items and services (Ebook 1 vs. EBook 2 - if the prices differ, the profit of each conversion would different and you're bidding on both), so the best route would be to compare them to each other. If the entire account is for only one service and sign up is the same on every landing page for every ad or keyword, then you'd want the conversion tracking.
Before setting up your AdWords conversion tracking:
You need to make sure you have a confirmation or message page that every visitor who fills out the form would see. The URL for this page is your "goal" - this is what you have to set up in your AdWords account.
This video covers it, but it's quick - you'll want to set up a goal as a "Lead" if I understand you correctly:
YouTube - [Advertiser Education] Conversion Tracking & Bid Management
Here's more detailed info:
How to Set Up Google AdWords Conversion Tracking
If you continue to see ads that has a high CTR but no conversions after a couple of weeks, you may want to look at other factors such as landing page layout/load time/etc., contact form length, ad text relevancy to landing page, etc. etc.
Good luck,
Danielle