Diff 2 trees recursively
diff -r dir1 dir2
Diff 2 trees recursively showing only filenames
diff -rq dir1 dir2
Diff 2 trees recursively showing only filenames of files which changed (don't show files only in one tree
diff -rq dir1 dir2 | grep -v Only
Diff 2 files side by side
diff -y file1 file2