server tasks - are ePO internal tasks. depending on the task, some could run once a day, some runs multiple times in a day. it can include downloading latest updates, sending automatic email to helpdesk or administrators, replications, synchronizations with active directory or vcenter etc. as you can see these do not affect your users / client systems. some server tasks are built in but you may need to configure or create your own.
client tasks - are what you run on users workstations. most common is DAT update. and when needed, you can upgrade products . for instance VSE 8.8 patch 7 to VSE 8.8 patch 8 upgrade, or deploying Site Advisor to client systems. you can of course remove a product which is also a type of client task. You need to define client tasks in the ePO client task catalog.
client deployment tasks will cause ePO to "push" packages to client/s. if network bandwidth is a concern, you should target a smaller batch of systems, tag them and schedule your tasks.
FYI - ePO 5.3 product guide (see chapter 17)