A mystery shopper is first and foremost an anonymous evaluator of a location according to specified criteria. The criteria are set forth in our instructions for the job. The reports filed are answering regarding conformance to the criteria.
Mystery shops can be as simple as evaluating the greeting and closing and promptness in the drive-through lane of a bank or as complex as evaluating your room, every service offered, every public space and every employee interaction in an upscale hotel or resort. Reports can be anywhere from a few Y/N/NA questions to a 25 page collection of questions with one sentence statements for every "No" and about 25% of the "Yes" plus about 15,000 words of narrative.
Payment for reports is generally on the lean side, out of pocket expenses to be reimbursed on acceptance of the job can be from about $1 to get a receipt through $1000 or more for an upscale hotel.
Most of us work on jobs that are between the extremes, generally with a 'value' of the shop (fee + reimbursement) of $10-20. Think of any chain of stores, restaurants or banks in the country and there are likely to be 'shops' done of them.
If you have read through the 'sticky' threads of this section it gives you a pretty decent notion of how this works, what you need to do to get organized and started and what companies are generally best to get started with to 'hone your skills'. There is more similarity between observations for shops than there is dissimilarity, so basic training makes sense. Once you have some simple, basic shops under your belt, the additional requirements for more complicated shops become small additions to your repertoire.