The specific difference between a senior editor and a managing editor can vary, depending on the hierarchical management structure in a particular business; indeed, some companies may even use the two terms interchangeably. In general, however, a managing editor is a higher position, and may be someone who is head of the editorial department. A senior editor typically represents someone who is in a high ranking position in the editorial department, but still reports to the managing editor. In a corporate environment, it might be easier to consider the difference between the two as supervisory and managerial roles, respectively.
Some businesses will arrange their workplaces in this way for a number of reasons. A clear division of responsibility and chain of command can help larger departments to function better, and it also allows people in higher managerial roles to focus more on management duties rather than supervising other staff. A senior editor and a managing editor will often work very closely together, and they will often share a workload when working on a large project. A senior editor will typically report to the managing editor, whereas the other people in the department such as copy editors or associate editors will often report to the senior editor.
Both types of editors are typically people who have a great deal of experience in their jobs. Most will have bachelor's degrees at the very least, if not a more advanced degree. Many upper level editors start out in the field working as copy editors, fact checkers, or proofreaders and will eventually work their way up into higher level positions with more responsibility. These positions might include line editing or content editing. Years of experience will often translate into promotions into senior or management positions.
In general, a managing editor will have the final say before a produced product goes to print. He or she may also be responsible for meeting with people such as authors or literary agents, depending on the nature of the editorial work. The senior editor might have different responsibilities, such as making sure that the various projects the editorial department is working on stay on track and are completed by the deadlines. He or she might be responsible for enforcing specific deadlines for employees throughout the department, and resolving any issues or answering any employee questions. Only the most challenging problems will be given to the managing editor, whose focus will be more on the "big picture."