Basic items
Items with the word 'basic' in their names can be used by almost any pilot and require only minimal skills to fit. To manufacture basic modules, you need a blueprint for the item and the minerals listed on it. No special skills or items are required. Eg. Basic Power Diagnostic System

Tech I
With slightly better attributes than basic modules, Tech I modules usually require a little training before they can be used. Their manufacture still doesn't requires special skills or items, however. You can tell most Tech I items by their names, since they end in 'I'. Ships however won't have the 'I' in their names. You can tell Tech I ships from Tech II by the Tech II label on the icons of Tech II ships. Eg. Power Diagnostic System I

Tech II
Tech II items are significantly better than Tech I items and most require extensive training for a pilot to be able to use them. Most Tech II items are better than their named Tech I counterparts (see below), except for many faction variants. Most Tech II items require special skills to be able to research or manufacture using a Tech II blueprint. And building Tech II items requires special parts in many cases.

You can recognize most Tech II items by their names, ending in 'II', although ships don't follow this rule. Also, all Tech II items will have a Tech II label superimposed over their icons if you have them in your hangar. Eg. Power Diagnostic System II

'Named' and faction
All technology levels can have named variants of their modules. The only way to obtain named modules is to collect them from the loot containers dropped by downed npc's. They cannot be built, unless you obtain a special blueprint copy for them from an npc, which is a rare thing and isn't available for all named items.
Example of a basic named module: Alpha Reactor Control: Diagnostic System
Example of a Tech I named module: Beta Reactor Control: Diagnostic System

Special named items known as faction items will be dropped as loot from npc's belonging to specific factions. These items are sometimes superior to their Tech II counterparts. They are generally very rare and don't appear in the market browser, although the market search will find most of them and show you whether there are any available on the market. Often, these items are traded through the forums or on escrow.
Example of a npc dropped named module: Dark Blood Power Diagnostic System
Example of a npc awareded named module: Ammatar Navy Power Diagnostic System

Comparing variants
If you open the info window for an item, its variations tab will list all the variations of the module. Note that basic items will only list the basic item and its named variant, whereas the Tech I, Tech II and it's named and faction equivalents will all list eachother on their variants tab.

This is also the definitive way to tell if something is basic, Tech I or Tech II.