Lead Shortcode forms and Lead Group forms are types of opt-in forms that you can build with Thrive Leads. However, they are different in some aspects.
The main difference is that you can create multiple types of forms under one Lead Group, but only one type under Shortcodes and the display settings can be set differently.
You can find both the Lead Groups and the Lead Shortcodes in the Thrive Leads Dashboard:
Lead Shortcode Forms
A shortcode form looks exactly like the post footer form - you cannot create any other type with this.
Adding a shortcode to your content is fairly simple: you just need to copy the shortcode of your form and add it to your content in the usual edit mode, manually.
You can copy the shortcode directly from the shortcode's individual page, after you click "Edit":
Alternatively, you can copy the shortcode from the Thrive Leads Dashboard:
Next, paste the copied shortcode into the post or into the page you want.
The shortcode form will only appear in the content where you added it to - it won't be automatically added to other content.
Lead Group Forms
There are several different kinds of forms you can create under a Lead Group: from Ribbon, Post Footer, Widget, Lightbox and Slide in, to Screen filler Lightbox or Scroll Mat.
Unlike shortcode forms, they are not manually added to your content, but you can set while building where you want the forms to appear. This display setting will automatically apply to future forms.
You can find the above mentioned display settings by clicking on the cog wheel icon on the right-hand side of your Lead Group.
Read more about Lead Groups here.
To sum it up: the main difference between the two forms is the design (more possibilities with Lead Groups) and the way of adding it to your content.