Smallville is a show set in, well, Smallville, featuring a young Clark Kent (Tom Welling) AKA Superman placed in a present day setting. It is screened on the WB in the US and Channel 4 in the UK.
The show is much the same to the movies and comics that The Phantom Menace is to the rest of the Star Wars trilogy. Clark is the young Obi-Wan Kenobi, who is still unsure of the extent of his powers. Throughout the series, we see Clark's "gifts" (as his parents always call it) come to light, complete with sweet special effects. His X-ray vision means that he sees everyone's skeletons (so no under the clothes naughtiness can ensue), whilst my favourite is when Clark uses his super speed (still no flying yet). The rest of the world slows down in a Matrix style, whilst Clark continues to run at normal pace and do what he does, taking bullets, knifes, bee stings, handbags.
In another smack of Episode I, Clark is best friends with who we know will be the bad guy: the irrepressibly cool Lex Luthor (Michael Rosenbaum). Whilst early attempts at making the character seem as rich as possible fell flat on their face ("This Scottish castle was moved brick by brick to my estate here in Smallville"), there is a technique which does it every time. Lex appears to drive a different car every episode, or at least has a fleet of about 5 supercars, including a Ferrari and a Porsche which he drives very fast with very loud music. If that doesn't scream "I'm rich, I'm rich, I'm comfortably well-off!", nothing does.
The inevitable love interest is the confusingly named Lana Lang (Kristen Kreuk), causing no end of questioning for the first few episodes: "Did he say Lois? It sounded like Lois Lane?" I thought this name was down to the writers, but TenMinJoe informs me it was already formed in the Superman continuity. Knowing this doesn't make it any less annoying. Of course, a love interest is only a love interest if the hero can't have it (Buffy and Angel being the classic unrequited love saga), so she's attached to annoying jock Whitney. To bubble up the sexual tension, Lana is always saying things like "You're the best, Clark" whilst making with the pretty eyes.
There are other characters in the show, such as Chloe and Pete, who obviously try and fill the roles of Willow and Xander from Buffy the Vampire Slayer. They fail miserably as the writers inevitably leave them out for a whole show, with just a head pop-up around a door as if to say "I'm still here! Making money, for doing nothing!" I've been thinking very hard as to why this is. At first I blamed the writers for not making any the characters very distinct and so couldn't be bothered, but then realised that that was becasue we hardly ever see them, which is a bit of a vicious circle in itself! I then pondered that perhaps it was because there are simply too many characters. That's wrong too, because Buffy has a large number of characters too, and each one gets a decent amount of face time. After mulling over why that was (yes, I really need to get out more) it then hit me. The main problem with Smallville is that it needs too many storylines! It was more exciting in my head. Anyway, each episode demands an X-Files-style introduction of the latest bad guy, which takes up the whole first 5 minutes anyway. Then we need to see the bad guy do his dastardly deeds, which is another 5-10 minutes. This would leave just enough time to sort out the rest of the characters, if they ever stayed together. Unlike Buffy where we see the characters follow the same storyline and they all talk to each other in the library or wherever, the Smallville "minor" characters don't hang around with the lead characters, and saunter off somewhere. Clark will be chatting up Lana with his sensitive talk, then we have a completely different storyline for Lex and then we put in the bad guy stuff. Once you put that altogether, you've got your episode, leaving no time for the other guys.
I once heard Smallville described as "Buffy crossed with Dawson's Creek" which I think is unfair to all concerned. Dawson's Creek has its own psychoanalytical babble each week which is strangely watchable, Buffy has its unique blend of drama, humour and action. Both of these shows don't waste their characters to the viewers benefit, and no-one goes "Oh not him again" when they appear on screen (apart from Riley from Buffy, but I've mentally blanked that after months of therapy). Smallville suffers from this problem terribly, but only because it doesn't develop characters as it should. However, the unique character of Clark saves the show. He is as sensitive as any teen show character you could compare him to, which is in part helped by the acting of Tom Welling, but doesn't veer off into Dawson Leary whining or is put through the extreme paces that the evil Joss Whedon likes to put Buffy Summers through. He is a much easier character to relate to, mainly because you spend a lot of your time believing that he isn't a puppet being tested to the emotional limit, just a normal guy who happens to save people from certain doom in his spare time (he doesn't appear to like television. I haven't seen him watch it once.)
A good reason to watch it is to try and play spot the in-joke. There is always something (however tenuous), such as people saying "You're not a super man, Clark", song lyrics which refer to "kryptonite" and the team mascot for the Smallville Crows is a crow dressed in Superman's costume.
And if you still need a reason to watch Smallville, the actors/actresses (depending upon your preference) are pretty hot.
Quick note: In case you were wondering, the theme tune for the show is "Save Me" by Remy Zero. If you're playing along with the spot the in-joke game at home, you can find the answers here in the episode guides: http://www.tvtome.com/Smallville/index.html (warning, Will Robinson, UK spoilers ahead!)
Big hand shakes: TenMinJoe again for that Lana Lang information.