Denzin and Lincoln provide a definition of qualitative research in Chapter 1 of the Sage Handbook of Qualitative Research:
Qualitative research involves the studied use and collection of a variety of empirical materials--case study, personal experience, introspection, life story, interview, artifacts, and cultural texts and productions, along with observational, historical, interactional, and visual texts--that describe routine and problematic moments and meanings in individuals' lives.
The Sage Glossary of the Social and Behavioral Sciences details some important aspects of qualitative research:
The purposes of qualitative research are to explore, discover, construct, and describe phenomena experienced by people in specific contexts.
A strength of qualitative research is its ability to provide an in-depth understanding of humans and their circumstances. A potential weakness is the inability to generalize findings to other populations.
There are two main categories into which most methods of research will fall into--qualitative and qualitative. It's important to be able understand these two differing approaches.
Watch the video below and check out the Library's FAQ Qualitative vs. Quantitative Research to learn more.