Quantitative research can be thought of as the "what" of the experiment or study. It's primarily concerned with estimating the size of your main interest -- that's why quantitative methods use sampling and rely on a tight structure to obtain results. For example, if you were conducting a clinical study, the quantitative data would reflect the dosage of the tested drug and how often it was administered -- while both types of information can be tabulated, they aren't open to interpretation.
Quantitative methods include: experiments, observations, information systems data retrieval, and interviews and questionnaires. Experiments encompass activities inside as well as outside of the laboratory. For example, clinical trials, while relatively large in scope, are still considered experimental. While interviews are used as both quantitative and qualitative, as a quantitative method they typically exist as a standardized set of questions and nothing more. When studying groups, randomization is essential, and controls should be used. In other words, the outcome of the control group will be known whereas the group or groups being tested will have one variable that will affect the outcome -- these results will be compared against those of the control group.
If quantitative research answers the "what" of the study, qualitative research can be thought of as the "why" -- a process not as easily defined as that of its counterpart. Typically these methods take longer to implement, because humans interpret for qualitative research what machines interpret for quantitative research. In this way, data collection has multiple angles -- cultural motivation and impact, and what subjects do as opposed to what they say, to name a few -- all of which are tabulated and analyzed by the researcher.
Qualitative methods are broken down into three main types: in-depth interviews, document reviews, and general observation. These encompass a wide range of methods that include open-ended surveys, audio and video files, and documents not directly employed by the study itself. One popular qualitative research method is the ethnography; using this method, researchers compare culture and behavior while living and/or working within the studied environment.