If you want to see what transactions are executed by a user in a specific time period do the following steps;
Go to ST03. Select choose for analysis. Choose only one application server at a time in case you have multiple application servers. Choose time period of your choice.
In the next screen from menu choose GO TO–>PROFILES->-USER PROFILES. Here you will get the list on users who have worked on that application server.
Double click on the required user and you will get all the transactions he/she has executed.
In case you select TOTAL in step 1 and then follow the steps 2 and 3 you will only get the list of application server on which the user has worked and not the transaction details.
You can also use transaction code STAD
The surest way is through the T Code STATTRACE :
The only draw back is the SAP maintains the STAT files only for 48 Hrs by default after which it is over written :
STATTRACE -> Intial User = User Name -> Start Time -> Time Period 1440 -> Execute
For changing the default value of the max/stat
ST03N -> Expert Mode ->Collector and Performance DB -> Workload Collector -> Control Files