Which type of inferential and machine learning analysis is used to find groups of observations that are similar to each other?

A number of types of inferential and machine learning analysis are very commonly used in Big Data analytics:
• Cluster – Used to find groups of observations that are similar to each other.
• Association – Used to find co-occurrences of values for different variables.
• Regression – Used to quantify the relationship, if any, between the variations of one or more variables.