r - Difference between raster() and readGDAL()?

I am new in working with rasters and after having trouble working with them i'd like to know the difference between those two import functions.

for example i did this

  library(raster)
  s<-raster("random.TIF")

when trying to plot it, returns the correct image, but trying this:

s<-readGDAL("random.TIF")

returns a black image.

Answer

None of these functions return a colored image, black or otherwise. This

library(raster)
s <-raster("random.TIF")
s   

Returns a RasterLayer object (or perhaps a RasterBrick if there are multiple layers in "random.TIF"). This class (type of object) is defined in the raster package. You can visualize it with

plot(s)

This

library(rgdal)   
x <- readGDAL("random.TIF")
class(x)

Returns a SpatialGridDataFrame. This class (type of object) is defined in the sp package. You probably did plot(s) but that does not work well as that would probably show a 'black image'. An appropriate way to plot a SpatialGridDataFrame is

spplot(x)

sp and raster objects play well together, but they are different. A possible starting point is reading this: cran.r-project.org/web/packages/raster/vignettes/Raster.pdf