in what context?
In general:
1) adjuvant is given as part of a curative approach hoping to eradicate micrometastatic disease and increase the chances of a cure. for example: you take a stage III colon cancer out. you're worried that there might be cancer cells hiding somewhere. you give chemo hoping to kill them and increase the chances that your patient is cured and won't have a recurrence.
2) maintenance is given in the palliative setting hoping to maintain your response and prevent rapid regrowth of tumor while minimizing toxicity. for example: you give your patient 4-5 months of chemo for widely metastatic colon cancer and they respond well. You know they will never be cured so you "back off" on the intensity of the chemo to something a little more tolerable in the hope of not losing your response quickly.
Of course, whether you give adjuvant chemo for a certain cancer (same for maintenance) is all driven by data but it can be quite controversial in some situations.