A Transaction snapshot depicts a set of diagnostic data taken at a certain timestamp for an individual transaction across all Application Servers through which the transaction has passed.

They are the best way for troubleshooting performance issues.
The snapshot will show you which Tier the issue is occurring
You can then drill down into that Tier. From here you can analyze the transaction quickly.

There are several different kinds of snapshots

1. Periodic Collection - A snapshot is taken every 10 minutes for each business transaction even if no problems are occurring.
a. Found in the Application Dashboard under ''Transaction Snapshots''->''Periodic Collection.
b. To change the default settings go to Configure->Slow Transaction Threshold->Configure Periodic Snapshots
c.Change the minutes setting

2. When the agent detects slow, very slow, stalled or errors it will capture snapshots.
a. Found in the Application Dashboard->Transaction Snapshots->Slow and Error Transactions
b. When a diagnostic session is triggered a snapshot is captured.

When looking at the transactions there are blue and grey icons next to the time stamp. Blue represents a full call graph. Grey represents that only a partial sequence was gathered

Partial sequence graphs start at the point that the call sequence determined that there was an issue. i.e. Slow or with errors and may not include the initial steps in the sequence.