Micro Conversions are not a full conversion as in a user may not have made a purchase yet but they have done something that is valuable or may lead to a Macro Conversion. The user may have subscribed by email, created an account or done some other type of event.
Inbound Marketing focuses heavily on Micro Conversions. These conversions can happen as the user interacts with your inbound marketing program.
Ecommerce can utilyze Micro Conversions. Lets say a customer starts an order then goes to checkout. During the process the user may have created an account but then for one reason or another did not check out. If you utilyze an abandoned cart program then you may send them an email reminding them that they left items in the cart. If they return via that email then that would be a Macro Conversion.
For a downloadable item one can declare an on-click event as a Micro Conversion.