There are a number of records that relate to our ancestors that were unfortunate enough to need to be admitted to the workhouse.
Firstly there are the admission records, which every poor law union kept of those admitted to the workhouse. They would include the details of each individual and the reason for their addmitance.
Other records would concern the infirmaries that were a part of the workhouse in almost every town.
The 1834 New Poor Law was never going to erradicate poverty and if anything it was more concerned it keeping the poor out than taking them in, with an attitude across all it's institutions that the poor were shiftless and to blame for their own poverty stricken situation.
Any surviving records would be in the County Record Offices, these will be listed in the CRO caralogue, but it is worth noting that the workhouses were always included in any census return, and so it might be wortth looking at the census first.
people were not necessarily long term residents in a workhouse, there as overcrowding and lack of funds to keep the really deserving fed let alone the shirkers.
Online I would imagine you would find some detailed lists of inmates, certainly some of the london workhouses have had their admission registers transcibed and put on various sites.