UID stands for Unique Identifier.  This is a unique number to identify, for example, images, reports, etc. Some common UID's as used in imaging are:

Implementation class UID: identifies a particular device type, exchanged during the DICOM Association negotiation

Study Instance UID: typically created by an information system to be provided in a DICOM Modality Worklist (MWL) to be used to identify studies uniquely

Series Instance UID: generated by a modality to identify a group of images from the same modality and equipment

SOP Instance UID: Uniquely identifies a DICOM object such as an image, generated typically by a modality or workstation

SOP Class UID: identifies a SOP Class such as "CT Storage SOP Class"

Transfer Syntax UID: identifies a specific transfer syntax such as "JPEG compression"