Graphical display of data helps to form preliminary hypothesis, observe patterns, etc. However, one should be careful in staring too long at a graph. In the words of Carl Sagan,

Humans are good, she knew, at discerning subtle patterns that are really there, but equally so at imagining them when they are altogether absent

This post is specific to the use of the R language for data analysis. Often, we need a quick look at the data in terms of summary of key variables. The function str() tells us about the dimensions of the data frame in terms of the number of observations and the number of variables. Further, it tells us the name of individual variables, their type (class) and list of first few observations.