Telling them apartEdit
You can tell a module is an active module by its attributes. If a module has an "activation time / duration" attribute (and generally, an "activation cost"), it is an active module. Modules that don't have these attributes, are passive modules.
If you are able to fit a passive module - that is, you have enough free CPU and power-grid to turn it on - its effects will apply immediately. The needed CPU and power will be subtracted and the bonuses and penalties of the module applied.
If, as a result, your ship no longer has the power or cpu it needs to keep all the modules online, it will throw modules offline until it does. This can be the case where a module actually lowers your available power-grid or CPU, like when you have Miner Is fitted and you then fit a Mining Laser Upgrade I.
Whatever passive modules are online will affect your ship continuously, without consuming cap or requiring further activation. However, there may be stacking penalties involved, but that's too complicated to discuss right here.
Fitting active modules is similar to fitting a passive module, the same rules to get it online apply. However, most of the attributes of an active module only come into effect if you activate the module. It's fitting requirements (Power-grid and CPU) apply immediately, as well as any explicitly passive effects, like on the Armor EM Hardener I for instance.
To activate a module, you need to undock. In space, the module will be represented on your HUD and by clicking the module once, it will activate, providing you have a sufficient amount of cap. The cap will be subtracted and the module will start its cycle. During the cycle its effects apply. Once the cycle ends, it either autorepeats (if you've set it to, right-click the module in space to check) or it stops and you have to click it again.
Some modules only take their effect once, at the beginning or end of a cycle, but you cannot activate them again until they complete a cycle. For instance, the Small Shield Booster I boosts your shield strength at the start of its cycle and the Small Armor Repairer I increases your armor strength at the end of its cycle.
Modules with both passive and active effects affecting the same attribute apply either the passive or the active bonus. Activating the module forfeits the passive bonus and applies the active bonus, until the module deactivates again.