Launch conditions specify when the workflow must be executed.

• Only on the first save: Triggers the workflow when you create a new record. For example a welcome email would have this launch condition so it gets sent ONLY when the new contact is created.
• Until the first time the condition is true: Triggers the workflow only once for a record that complies with the defined conditions. Once executed, your workflow will not trigger on the same record again. However, it will trigger actions on other records in the selected module when they fulfill the conditions.
• Every time the record is saved: Triggers the workflow every time you save a record, including the first creation save.
• Every time the record is modified: Triggers the workflow every time you edit and save your record, EXCEPT on the first creation save.
• Scheduled: Triggers workflow on a certain date/time. Time based workflows.

Complicated? Umm, let me give you a few examples for a better understanding

Until the first time the condition is true

  Records with 'Sales Stage' equals to 'Closed Won' should be automatically assigned to me. This happens only once in the sales cycle of a record. For that reason, I will create a workflow and have the condition Until the first time the condition is true. Once the workflow is executed, it's not going to perform the same operation on that record again. 

Every time the record is saved

  When a deal is won, 5 % share of the total amount goes to the sales agent. I do not waste my time in calculations. I would rather create a workflow. This should execute every time record is saved; as a result, values will be automatically updated. 

Every time the record is modified

  I want vtiger CRM to trigger me an email if someone modifies information in 'Amount' field in Opportunities module. I will create a workflow and have the condition Every time the record is modified. This workflow will execute every time edit+save operation will be performed on a record.