Phonetics are how words sound. A written "g", for example, has a "guh" sound, like in "good". But, since this is English, it can also be silent (like in "tight") or soft (like "genre"). And that's just one letter of 26. Fun, right?
Part of the reason for this is that although the English language developed from a Germanic base (the ancient German language, not the modern one) on an island nation, those people frequently got conquered or resettled, bringing new languages with them, whether the Indo-European languages (like Celtic), the French, or the Romans' Latin. Linguists often fight about how it happened, but that's the gist of it.
Basically, phonetics is useful only to a point in English, unlike in our sister language, German, wherein phonetics is fairly straight-forward. Words sound the way they should in German.
A good example of this is the River Thames, the river that runs through London and whose name I horrifically butchered while teaching last week. If we used basic English phonetics, the pronunciation would sound like it looks, a "th" sound followed by a long "a" (like the "a" in "gain"), then a "mmmm" and "ssss". But, this being English and the word coming from a possibly Celtic origin, the word actually sounds like "Temz". I mean, seriously, really? What a pain in the butt.
To the left, you see Psylocke. She's a comic book heroine, an X-Man, but has a history that nicely fits with this issue. You see, she was born British. But as you can see above, she's a Japanese ninja. This is due to a body shop wherein her soul was switched with another character. Much like the English langauge, Psylocke retains her British accent when speaking (her twin brother is the character Captain Britain, actually, which just reinforces the irony) but still has a very different appearance, much like many English words who look one way and sound completely another.
And what's with her name in the first place? She's a psychic, so that's the "psy" part. And being British, the "locke" is most likely for the British philosopher John Locke. But when I was a kid and started reading comics, I tried using my superior phonetics to figure out her name, and somehow came out with something like "Pi-low-ki". Amazing kid, I was. It wasn't until a few years later when Psylocke showed up on the 90's X-Men cartoon that I realized how wrong I was.
So what's the lesson here? Basically, be careful of English pronunciation. In general, you can actually never know how something's pronounced until you hear somebody else say it.