What is the difference between echo and print?

1. Speed.  There is a difference between the two, but speed-wise it should be irrelevant which one you use. 
 
echo is marginally faster since it doesn't set a return value if you really want to get down to the nitty gritty.

2. Expression.  print() behaves like a function in that you can do:
$ret = print "Hello World"; And $ret will be 1.  That means that print
can be used as part of a more complex expression where echo cannot.  An
example from the PHP Manual:
$b ? print "true" : print "false";
print is also part of the precedence table which it needs to be if it
is
to be used within a complex expression. It is just about at the bottom
of the precedence list though.  Only "," AND, OR and XOR are lower.
3. Parameter(s).  The grammar is: echo expression [, expression[,
expression] ... ] But echo ( expression, expression ) is not valid.
This would be valid: echo ("howdy"),("partner"); the same as: echo
"howdy","partner";   (Putting the brackets in that simple example
serves
no purpose since there is no operator precedence issue with a single
term like that.)
So, echo without parentheses can take multiple parameters, which get
concatenated:
echo  "and a ", 1, 2, 3;   // comma-separated without parentheses
echo ("and a 123");        // just one parameter with parentheses
print() can only take one parameter:
print ("and a 123");
print  "and a 123";