Derick Rethans has a new post to his site introducing the aggregation framework that's a part of the MonogDB database system. The aggregation framework gives you functionality to access aggregated values from queries without having to map/reduce.
I wanted to find out which cuisines are used in all of the documents of my dataset. Described in a different way: I want all the different cuisine=... tags as used in my dataset. Traditionally you would write a really complex™ Map/Reduce job, but since MongoDB 2.2, there is a new feature called the aggregation framework. The aggregation framework is meant to be an easy way to do fairly complex aggregation jobs.
He talks about the various keywords and features the framework provides like "$match", "$project" and "$unwind". There's also code snippets included that show both the Mongo command line format and the PHP MongoClient format for making the queries. For those more familiar with writing RDBMS SQL, this functionality might help to replace some of the concepts you've been missing.