Final answer:
Exploratory research is employed to explore phenomena that are not well-understood and to investigate little-known phenomena, differing from longitudinal and cross-sectional studies which track changes over time or provide a snapshot of a phenomenon, respectively.
Step-by-step explanation:
The type of research used to explore phenomena that are not well-understood and to delve into little-known phenomena is exploratory research. This form of research is generally used when researchers are beginning to understand a topic or when the subject of study is not well-established. Exploratory research allows for the investigation of questions that have not been precisely defined yet and provides insights that can form the basis for further, more detailed studies. In contrast, longitudinal research involves following a group of research participants for an extended period of time to observe outcomes and changes.
Observational studies are often employed in cases where experimental studies are not possible or ethical. They measure characteristics and seek to establish associations without the manipulation of variables. Cross-sectional studies are a specific type of observational study that collects data just once at a certain point in time, providing a snapshot of a specific phenomenon.