Do

Do[expr,n]

evaluates expr n times.

Do[expr,{i,i_max}]

evaluates expr with the variable i successively taking on the values 1 through i_max (in steps of 1).

Do[expr,{i,i_min,i_max}]

starts with i=i_min.

Do[expr,{i,i_min,i_max,di}]

uses steps di.

Do[expr,{i,{i₁,i₂,…}}]

uses the successive values i₁, i₂, ….

Do[expr,{i,i_min,i_max},{j,j_min,j_max},…]

evaluates expr looping over different values of j etc. for each i.

Details

Do uses the standard Wolfram Language iteration specification.
You can use Return, Break, Continue, and Throw inside Do.
Unless an explicit Return is used, the value returned by Do is Null.
Do[expr,Infinity] continues evaluating expr until explicitly told to exit through a function such as Break, Return, Throw, Abort, or Quit.
Do[expr,spec] first evaluates spec, then localizes the variable specified and successively assigns values to it, each time evaluating expr.
Do effectively uses Block to localize values or variables.
Do[expr,spec₁,spec₂] is effectively equivalent to Do[Do[expr,spec₂],spec₁].
Parallelize[Do[expr,iter]] or ParallelDo[expr,iter] computes Do[expr,iter] in parallel on all subkernels. »