What is the purpose of promoters and terminators in gene expression?

These are both used for the initiation and termination of transcription, respectively. Promoters serve as binding sites for RNA polymerase, the enzyme used to synthesize mRNA (that's later translated into protein).

But, RNA synthesis can't go on forever, that'd be pretty wasteful -- think of how long your messenger would be if RNA polymerase kept going until the end of the DNA molecule. Not all of that would be usable, translatable sequence. So, you have to include a terminator, a sequence on the DNA that tells RNA polymerase to stop making RNA.

The CMV 35S promoter is a promoter from cauliflower mosaic virus. It's a virus that normally infects cauliflower (hence the name), It's used to make an RNA molecule of 35 Svedberg units (35S) in the normal virus. Genetic engineers have isolated this promoter, since it's really strong (high affinity for RNA polymerase) and is constitutive (the gene will always be "on" in the cell). So, if your gene is being driven by the CMV 35S promoter, it will be transcribed a lot all the time.

The NOS terminator is the terminator region of the gene for nopaline synthase. Nopaline is an opine, a food source for Agrobacteria made by plants (albeit using a gene transferred from the Agrobacteria).