Minecraft – the difference between Tekkit and normal Minecraft

It is the wu.class causing the error. Both IndustrialCraft 2 and Zeppelin need a different version of it. Try merging the wu.class in the tekkit jarfile and the wu.class in zeppelin mod.

I'd say that the most major difference is the addition of the Thaumcraft and Mystcraft mods in Technic. As others have said, Tekkit is for use in a multiplayer setting, while Technic is for SSP only.

The major mods in both are IndustrialCraft2 (adds electricity and industrial machines), Equivalent Exchange 2 (adds in alchemy using the Philosopher's Stone, and several highly powerful end-game tools like the Void Ring, Ring of Arcana, etc), Redpower 2 (adds in redstone wiring and machines), and Buildcraft (adds in some automated mining machines as well as transport tubes and controls for that).

In addition to those core mods shared by both packs, Technic adds several features that are best in singleplayer. Major differences are:

• Thaumcraft / Thaumcraft 2: Thaumcraft is a magic and arcana inspired mod for Minecraft. It features aura and vis (pronounced "vees"), which introduces a new mechanic for enchanting and crafting unique items. Thaumcraft also introduces the "Taint" - if you've ever played Terraria, the Corruption from that game is similar to the Taint in Thaumcraft.
• MystCraft: MystCraft is a mod for Minecraft inspired by the Myst series of video games. It allows the player to craft different types of books which allow for teleportation between different "ages", which are basically new worlds. As you get more advanced, you can write your own books, which lets you customize the ages to your liking. For instance, I wrote an age with the "Hell" modifier, which spawned me in a world with a ton of Eldritch Obelisks (artifacts of great power in Thaumcraft), which helped me to get ahead on my research.

A few other minor changes worth mentioning:

• Somnia: Simulates the world while you're sleeping. So, unlike vanilla Minecraft, when you sleep, stuff smelts, crops grow, etc.
• Mo' Creatures: Basically what it says on the tin. There's a ton more creatures in Technic than there are in regular Minecraft.
• Treecapitator: Handy little mod so that, if you chop the bottom of a tree out with an ax, all the wood in the tree falls down at once (kind of like real life, eh?).