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next, continue, nextfile
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Ia. next is a powerful command in awk. next reads the next line and control
jumps to the TOP OF THE PROGRAM. in contrast for "continue"  the control
simply goes to the top of the loop (in which continue is embedded).

here is a typical example of the use of next

awk -F: 'NF==1{print >"b"; next}; NF==2{print $1>"a"; print $2>"b"}' infile 

the infile has the form
	val1:vall2
	val1
	val1
	val1:val2 (and so on)

The idea is to stick write all val1 to "a" and val2 to "b". When NF=1 then
we write to file "a" and there is no need to worry about writing to file
"b". So next saves additional checks. with clever use of next you can 
improve the efficiency of your code.

Ib. Another use of next is to prevent other actions if one action is take.
This is not a matter of efficiency but of necessity.

Say, you have a file "a" which calls for other files with lines starting
with "\include{FILENAME}"

$ cat a
This is Hello Kitty
\include{parvi}
I am from Japan
\include{rusty}
I have heard of rabbits from the US

Your goal is to write an awk script which prints normal lines but
which print the contents of files when the program encounters "\include{FILE}"

$ awk '/\include/{gsub(/^.+{/,"");gsub(/}$/,"");system("cat " $0);next}{print}' a

So when a line with "\include" is found you wish to print the contents (you
leave the heavy lifting to cat). But you also wish to print the input line
for other lines. The use of next simplifies {if "/include" cat else print
input line} logic.  In fact the above one-line can be simplified to
$ awk '/\include/{gsub(/^.+{/,"");gsub(/}$/,"");system("cat " $0)}1' a      
 

nextfile closes the current file and starts reading from the next file on 
the stack.