One of the most important things in SharePoint 2013 is cross-stage workflows. SharePoint 2010 has "sequential" chain-like workflows with plain passage in flow
Workflows in SharePoint 2013 can be represented as shown in a schema:
The flow can pass from one stage to another. As you see, flow passes from Stage 2 to Stage 3 if Condition 1 is satisfied, and from Stage 2 to Stage 4 if Condition 2 is satisfied.
In SharePoint Designer you can develop workflows without any line of code.
|SharePoint Designer 2013 workflow, text-mode example|
In text mode you just add a stage, define it actions and set it transition to any stage. Stage with transition to workflow terminating must exist.
|Stage transition explanation|
In transition gate you can define conditions and so determine stage to go:
|Gate with condition|
And as a bonus, a Visio-like workflow view is available if you have Visio 2013 installed. So you can develop workflows in a graphical mode:
Happy workflow developing!