System Sequence and Activity Diagrams

During that last few weeks we've been talking a lot about system sequence diagrams and activity diagrams for our semester group project.  For me these always seemed pretty similar and I remained confused about them until I decided to make research them a little more for excellent blog posting material.  So here's what I could find out about Sequence System Diagrams and Activity diagrams.

Both diagrams deal with making a use case more visual to be easier to understand at a glance.  Both deal
with the events going on between the various actors and/or systems

The difference between activity diagrams and system sequence diagrams as far as I can tell is this:

System Sequence Diagrams
System sequence diagrams deal specifically with the system that you're working with recording the actions made by the actors on the system that may or may not cause a response from the system.  It deals with the  chronological order of interactions of the system and actors.  It must include the system with which you are dealing with as an object.  System sequence diagrams only deal with actions and reactions with the system.

I think the clearest most succinct explanation of Systems Sequence Diagrams came from a website I found (source 2). It said:"The UML system sequence diagram (SSD) illustrates events sequentially input from an external source to the system. The SSD will define the system events and operations. System sequence diagrams are a time line drawing of an expanded use case."

Some other ideas/facts about System Sequence Diagrams:
The objects are brought in when they start.
Not all the lifelines will be the same length

Activity Diagrams
An activity diagram as far as I could tell is different in that it does not require a system to interaction with. Also an activity diagram can be between 2 people or 1+ persons and a system.  Activity diagrams don't necessarily need to be in chronological order though most times it seems like they are as that tends to be a more natural way of thinking through order.  Activity diagrams can include the documents that are being used in use case and show their flow with the activities.

The clearest most succinct explanation of Activity Diagrams came from a different website (source 5).  It said the following: "Activity diagrams are typically used for business process modeling, for modeling the logic captured by a single  use case or usage scenario, or for modeling the detailed logic of a business rule......In many ways UML activity diagrams are the object-oriented equivalent of  flow charts and  data flow diagrams (DFDs) from structured development. "

Now for a more comprehensive understanding of the different symbols used in both and how to create them I will leave to further study by you.  I would recommend starting with some of these sources.  They could get you started on the right track.

Sources:
1. http://www.comptechdoc.org/independent/uml/begin/umlsyssequence.html
2. http://answers.yahoo.com/question/index?qid=1006022103106
3. http://vinci.org/uml/diags.html
4. http://en.wikipedia.org/wiki/Sequence_diagram
5. http://www.agilemodeling.com/artifacts/activityDiagram.htm