Is there any difference between sale and lead tracking?

For sales and leads you can use the same code - sale tracking code. The difference is, that when you track a lead, there is no Total Cost. If you want to track leads, you should place the sale tracking code into the thankyou page that appears after successful form submission (e.g after signup).

Note: If you use a sale tracking code to track a lead, in Post Affiliate Pro the transaction type will say "sale". In case you wish that to be changed, you should use the Action Commissions feature and a 'lead' tracking code instead of a sale tracking code. More info here