An interactive session is, in research cluster parlance, a scheduler job where you are free to enter run commands the same as if you logged into the server via SSH. If you're developing code in the cluster environment, want to check your code, or just run some commands on a cluster node this is the option you want.
The job will stay open similar to an ssh session, that is if you disconnect the session the job is over. This limitation can be circumvented through the use of programs such as screen or tmux on the login node.
To run a simple interactive job on the cluster use the srun command as shown below.
srun --pty bash -i
Running the above command on the login node would start an interactive job with the default limits. After running this you would then be running from one of the three gpu nodes with one core (and no GPU) allocation for a length of five days.
srun --pty -c 2 --nodelist=ax08 --gres=gpu:gtx1080:2 -t 0-01:00 /bin/bash
In the command above the parameters are a follows
--pty says to start a pseudo terminal interactive session (mandatory for interactive sessions).
-c 2 means use 2 cores (optional, default is 1 core)
--nodelist=ax08 means only run on the node named ax08 (optional, default is any to run on any available node)
--gres=gpu:gtx1080:2 is to use 2 gpu boards of the gtx1080 variety (optional, by default no gpus are allocated, note allocated gpus will appear starting as id 0)
-t 0-01:00 is to set the time for the session, in this case 1 hour (generally, the shorter the time you request, the higher the likelihood your job can be scheduled, this will be come more important as the cluster fills up!)
/bin/bash is to run the program you wish to start in the interactive session, the bash shell (mandatory, you need to specify a program it doesn't have to be a command line interpreter like bash it could by python or matlab if that's what you want to run)