Research Design
In general, the choice of a research strategy is determined by: 
  •  the type of research question,
  •  the available means, and 
  •  the units of analysis. 
Most introductory text books do not distinguish clearly between research designs and data collection methods. Research designs (used to explain things) can be categorized as:(quasi-)experimental, and as non-experimental. 

(Quasi-)Experimental Designs
Experimental designs involve at least two groups, administering a treatment and observing the consequences. There are various subcategories exist here, depending on:
  •  How many groups are examined (e.g., two-group design, factorial design, ... etc.)
  •  How subjects are allocated to different groups.
  •  How many times the groups are observed. 
Experiments (consider random assignment) are generally the best way to make valid causal inferences. However, many research questions are not susceptible to experiments. 

Non-Experimental Designs
Non-experimental designs involve making observations with a single group. Various subcategories exist, and this is depending on:
  •  How many cases are studied (Quantitative).
  •  The number of time points at which observations are made.
Quantitative, non-experimental designs can be used to make accurate descriptive inferences about a population. At this point it means they are strong at external validity. The advantages of Qualitative research are:
  •  considered to be a best approach to gaining insights into a topic with a view to developing hypotheses (exploratory research). 
  •  that used to provide tentative tests of hypotheses on research questions that cannot be answered using experiments or quantitative non-experimental designs.
An extensive overview of qualitative designs can be found here.