What is  a Spoonerism?
Transposition of initial consonants or sounds in a pair of words
Also known as "slip of the tongue".
Perhaps the following might help you get some of the feeling for spoonerisms -- they're one of Ted's favourite forms of wordplay. 
It's not simply a matter of the 'Transposition of initial consonants', though that may often be the case as in 'cat flap' / 'flat cap' or 
Soul of Ballad and Bowl of Salad. 
Spoonerisms are meant to be 'heard', not 'seen'.  Not like children!

           " Listen to your old pal TED.
            Spoonergrams are best when SAID.
            They lose their charm when they are READ". 

Note the different spellings for the same SOUND in the above, but note the different sounds for the similar sets of letters in the following words:
            TEDious (teed); Port SAID (side); READing matter (reed).
 

What is a Spoonergram?
This is basically a sentence containing a number of spoonerisms.
involving the process of swapping letters, sounds or syllables, which is known as metathesis. 
The following is a short example of a spooner- gram, based on one of the most widely known spoonerisms:-
What the cyclist is doing, then music's ensuing, biking up the strand, striking up the band!

More references to Spoonerisms can be found in Ted Clarke's new book "HOW THICK IS A SHADOW?"