If you come from another language, you might be tempted to use the += operator when appending to a string.

my_str = "Marco "
my_str += "Polo"

It works… but there is a better way to do it : the << method.

my_str = "Marco "
my_str << "Polo"

#UPDATE
I have removed my initial claim about operator precedence as it isn't really accurate. There is a much better reason to use << instead of += when appending to a string... something I didn't even realize (Thanks to Gregory). += will create a new String instance and will assign it to your left object. On the other hand, << will append the new string directly into your already existing left object. One could argue that += is only different than << and not better, but to me << is what you'll want to use 99% of the time (if not 100%) when appending to a String. I have yet to see a real case where one would want to "lose" it's existing String instance just to get a new one containing the exact same value. You can also use the << method with arrays :

[1,2,3] << 4 #gives [1,2,3,4]

Be careful however when using << with Fixnum/Bignum instances. With these objects, the << method will shift the bits of your integer to the left. If you want to do a summation (append style that is), you will have to use +=