As explained in the documentation (read the Getting Started chapters and search for "callback"), obj is a handle of the calling object, e.g. the figure or a uicontrol. The variable event is a struct, which explains the event, which has triggered the callback. The contents of the variable depends on the type of the callback, e.g. an activated UICONTROL replies another event description as a CellSelectedCallback of a uitable.
You can set a breakpoint in the callback and inspect the contents of the event variable in the command window or the workspace browser to learn more. Because the event struct has been the empty matrix in old Matlab versions, you can get nearly all possible actions and informations just by using the object handle and its properties.
