Hi TVDC,

I'll try to help out as I was also very confused about this when I first started working with Odoo v8. 

The short answer is:

1) You can use  "HTML" and edit the template via the visual WYSIWIG editor. To do this, locate the invoice you wish to modify and change the default from PDF to HTML.  Now open ANY invoice. Select Print > Invoices (or whatever you named your invoice). From here a new window will appear and an "EDIT" button will appear in the upper left. Click EDIT and change what you wish. I personally found this far too restrictive to accomplish what I wanted. I wound up with option #2.

2) Option #2 is to use QWeb. QWeb is Odoo v8's new templating system. To do this go back to Settings > Reports > Invoices (or whatever the name of your invoice is) and select "Search QWeb Views". There will be two options here. Select the one with the suffix _document 
On this screen you can edit the raw HTML / XML as you see fit.

You will see lots of talk around these forums of RML. My understanding is that this is Odoo's OLD method of modifying invoices. You should ignore everything other than QWeb or editiing it via the WYSIWIG editor. (The two options I listed above)

I have tried to include more detail in another post where I had a similiar question, here:
https://www.odoo.com/forum/help-1/question/modifying-invoice-templates-creating-multiple-invoice-templates-in-odoo-v8-71588