Special Attack: 55
Special Defense: 80
Scyther boasts the highest total base stats of any Pokémon that can evolve. Additionally, Scyther also has the same base stat total as its evolution, Scizor, the only Pokémon with such a distinction — generally, a Pokémon's stats increase as it evolves, whereas they're simply distributed differently between Scyther and Scizor. (Mega Scizor finally gives it a bump, up to a base stat total of 600.)
These high stats, along with the introduction of the Eviolite in Gen V, make Scyther a viable Pokémon in its own right, though its 4x weakness to Rock is a major downside.
Scyther caught in the wild come with one of two abilities, and have a Hidden Ability as of Gen V. (For more about Hidden Abilities, see Bulbapedia.)
Swarm is Scyther's first ability. When Scyther has ⅓ of its max HP or less, Bug-type moves do 1.5x damage. While this sounds useful, you don't really want your Scyther to have ⅓ or less of its HP, and it doesn't have many great Bug-type moves to choose from.
Technician is by far Scyther's best ability, and strengthens moves of 60 base power or less or 1.5x their power. This makes moves like Aerial Ace and Bug Bite amazing STAB moves, where before they were useful but much less powerful. If you use a Scyther, you're going to want to use Technician.
Steadfast is Scyther's Hidden Ability, and raises Speed one stage whenever Scyther flinches. Given that you can't control when your Pokémon flinches, this isn't a very useful ability. Pass on it.
Starting with Gen II (Gold, Silver, and Crystal), Scyther can evolve via link trade if it's holding a Metal Coat. This causes it to evolve into Scizor, a Bug/Steel-type. As mentioned above, evolving into Scizor confers no stat boosts onto the Pokémon; evolution is purely for the type change and new movepool. As of Gen VI, Scizor can temporarily Mega Evolve into Mega Scizor during battle if it's holding the Scizorite, which does grant it an extra hundred points into its stats.
You can find the Metal Coat in various places throughout the games; Bulbapedia has a full list. The Scizorite is located on the third floor of the Frost Cavern, behind the Abomasnow, post-game.
Scyther is found in various locations throughout the games. Bulbapedia and veekun have full breakdowns, but one thing that's worth noting is that in some games, Scyther can only be caught in the Safari Zone or in the Bug-Catching Contest. This makes it particularly tricky to get a Scyther of your own, especially because Scyther appears very rarely in both locations. On the pro side, if you manage to catch one, you're likely to win the Bug-Catching Contest. That kid with a Weedle can suck it.
Scyther learns a lot of very useful egg moves. Unfortunately, as it's part of the Bug breeding group, it can be difficult to breed some of these moves, which means you may need to make use of chain breeding to get them. (I tend to use level-up and TM moves, personally, so I don't have much experience with Scyther's egg moves.)
If you're looking for a list of the moves Scyther can learn, I recommend veekun's Pokédex. There are links to Scyther at other Pokédexes at the bottom of that page, if you'd like other sources.