plot() is a generic plotting function that produces a variety of basic plots, depending of the class of its arguments.
Plots  

plot(x) 

plot(x,y) ) 

plot(y~x)  Using a formula (presence of ~) you can also specify plot(x~y,data=frame)

plot(y ~expr)  expr can be a single vector or an expression of the form X1+X2+X3; produces a scatterplot for each y,x combination (click the graph to view the next graph in the sequence). 
Also, depending on the type of data, plot() the plot may be adapted; for instance when you are using time series data, points will automatically be connected by lines.
data(sunspots) plot(sunspots)
The data() makes the sunspots data available (many datasets are included in R or in pacakges.