Service-Object-Pair (SOP) Classes and SCU/SCP are terms that define the DICOM services and their role, either client or server.

An SOP Class is defined as a combination of a DICOM Service Element, which is a command, and an Object, which is defined by an Information Object Definition (IOD).  

Examples of Service Elements are C_Store, C_Get, C_Find, C_Move, etc. Examples of Objects are CT images, MR images, but also include schedule lists, print queues, etc. For example, the SOP Class "CT Storage", is a combination of the C_Store DICOM Service element (or command) and a CT Image object. A device supporting the CT Storage SOP Class will generate CT images according to the Class definition, and use the Store command to exchange them.

An IOD can be compared with a template or form that is used to create instances of objects such as images. In metaphorical terms, imagine you just made a company trip and want to get your travel expenses reimbursed. In order to do so you have to get a certain form with a particular number, and for argument’s sake let’s say the form’s number is387-T. This form is the IOD, and its identifier (UID) is 387-T.