A spoonerism occurs when corresponding sounds (consonants, vowels, or morphemes) in two words in an expression are switched. The effect is often humourous, as in:

• 'The Lord is a shoving leopard'.
• 'It is kisstomary to cuss the bride'.
• 'Mardon me padam, this pie is occupewed. Can I sew you to another sheet?'

A malapropism occurs when words that sound similar are substituted for each other. Here are some examples:

• 'He's as headstrong as an allegory on the banks of the Nile.' (i.e., alligator)
• 'He is the very pineapple of politeness." (i.e., pinnacle)
• 'If I reprehend any thing in this world, it is the use of my oracular tongue, and a nice derangement of epitaphs!" (i.e., apprehend; vernacular; arrangement; epithets)

So spoonerisms and malapropisms are both linguistic phenomena that involve substitution, but of rather different things. Etymologically, both terms are derived from the names of people. Reverend William Archibald Spooner (1844–1930), Warden of New College, Oxford was notoriously prone to producing spoonerism. The examples above are attributed to Spooner, though many such examples are said to be apocryphal.

Conversely, malapropisms are named after a fictional person, Mrs Malaprop. She appears in Richard Brinsley Sheridan's play The Rivals (1775) who is noted for constantly committing malapropisms. Her name is derived from the French phrase mal à propos, meaning 'inappropriate', and is an example of an aptronym—a name that fits some aspect of a character.

Understandably, the humour that may be present in a malapropism only works if the audience understands and recognises the misused and intended words. So the examples above, all attributed to Mrs Spooner, might be a little highbrow for the present-day mass audiences. To remedy this problem, one can turn to some of the gaffes made by George W. Bush, which are discussed by 'the language guy':

• 'I am mindful not only of preserving executive powers for myself, but for predecessors as well.'
• 'Natural gas is hemispheric… because it is a product that we can find in our neighborhoods.'
• 'The law I sign today directs new funds… to the task of collecting vital intelligence… on weapons of mass production.'
• 'Oftentimes, we live in a processed world, you know, people focus on the process and not results.'

Sources

Answers.com

• spoonerism
• malapropism

Wikipedia

• spoonerism
• malapropism

Britannica.com

• Mrs. Malaprop