The term research design refers to the overall strategy that you choose to integrate the different components of the study in a coherent and logical way, thereby, ensuring you will effectively address the research problem; it constitutes the blueprint for the collection, measurement, and analysis of data.

A research design is the arrangement of conditions for collection and evaluation of data in a fashion which is designed to combine relevance to the research purpose with economy in process.

The function of a research design is to ensure that the evidence obtained enables us to answer the initial question as unambiguously as possible.

Key elements of a good research design include:
a. sources and kinds of information strongly related to the research problem.
b. strategy indicating which method is going to be employed for collecting and analysing the data.
c. the time and cost budgets because most research is done under these two constraints.

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Sources: 
http://universalteacher.com/1/elements-of-research-design/
http://www.nyu.edu/pages/classes/bkg/methods/005847ch1.pdf
http://libguides.usc.edu/print_content.php?pid=83009&sid=818072