Lightning Component/Component bundle

• They are the self-contained and reusable units of an App. Apps can be either a single component or group of multiple components.
• Components are standalone and can be resolved in client side (i.e. browser) without making iterative server calls.
• Component bundle consists of 8 resources. The most important ones are 
• Component:
• This is where you can build your UI using Aura tags.
• Controller:
• Just like Apex controller used in VF pages, we have controller for Lightning Components.
• These controller can perform all the logical task in the client side without any server calls.
• It is done using Javascript.
• They also acts as an bridge and can call the server Apex methods.
• Helper:
• The reusable code of your component can be built here.
• It is a best practice to write the code in helper so that the component is lightweight and the application flow is easy to understand.
• Style:
• Any custom CSS can be stored here. 
• Before building any custom CSS, have you seen the Salesforce Lightning Design System (SLDS). It allows you to build jaw dropping UI without any CSS!