Firstly, note that procedure can be either documented or not whereas all work instructions are documented. A documented procedure could be a Standard operating procedure (SOP) or a Test Method. A SOP documents a higher level of information to standardise a full procedure. It covers who is responsible for what, what and how activities are carried out and a certain amount of operational detail, especially if there are no separate Work instructions as appendices / attachments. An example would be a SOP for a particular test. This type of SOP would typically include several steps and tasks, such as preparing the sample, calibrating the instrument, analysing the sample, and reporting the results.
A work instruction on the other hand is used to document the detailed steps of how to perform a particular task, for example how to perform a daily verification of a balance. That said, bear in mind that documentation could include any media. Thus, a work instruction could be written out as text, a diagram, audio file or video; or a combination thereof. SOPs are typically written out as text, with diagrams and tables included.