: -- separate commands from each other
print "Hello ":print "World"
The colon (':') separates multiple commands on a single line.
The colon and the newline-character have mostly the same effect, only that the latter, well, starts a new line too. The only other difference is their effect within the (so-called) short if, which is an if-statement without the keyword then. Please see the entry for if for more details.
if (a<10) print "Hello ":print "World!"
This example demonstrates the difference between colon and newline as described above.