Lightning Experience / Lightning UI:

• Lightning Experience can be turned ON at the profile level, or for individual users using permission sets.
• ALL standard profiles are lightning UI enabled by default, when you enable Lightning UI for your organization. [Workaround: Replace standard profiles with custom profiles]
• Visualforce pages work as usual in the lightning UI, however you might want to do a sanity check to make sure nothing is broken.
• Visualforce pages open with the old look and feel. The old look and feel is referred to as Salesforce Classic.

Where to learn more?

There are tons of resources in trailhead. Collect your badges and learn the fun way.

https://developer.salesforce.com/trailhead/trail/lex_admin_migration

Lightning Design System:

Lightning Design system is a front end framework for developing web applications. Using the Lightning Design system you could design visualforce pages or standalone web pages that look and feel the same way as the Lightning UI. A few pointers.

• Lightning Design system could be though of as an equivalent to 'Twitter Bootstrap'.
• To download and start using visit https://www.lightningdesignsystem.com/
• The design system is purely made up of CSS files.
• The design system comes with pre-built components such as pop-up modals, tree structure, buttons etc.
• Lightning Design system focuses mainly on the front end. Data manipulation and processing will still be done using Apex and Visualforce/other desired markup language.

Where to learn more?

 Trailhead again. Use this trail to learn more https://developer.salesforce.com/trailhead/module/lightning_design_system

Lightning App Builder:

The Lightning App builder is a drag and drop tool to build lightning pages. Salesforce offers a few components by default, including 'Recent Items', 'Assistant', 'List View' etc. In addition you can create your own custom components using the Lightning Design system, or use third party components supplied by AppExchange applications like SKuid

Access from SetUp -> Lightning App Builder

It is important to understand how approval email notifications work in Salesforce.com, particularly when you are mass submitting/re-assigning approval requests. You might want to avoid unpleasant situations wherein users are flooded with email notifications in their inbox. This article tries to point out the various options available to avoid such situations.

How does Salesforce send an email to the approver when a request is submitted?

How to stop email notifications while mass submitting/reassigning approval requests?

With Visualforce email templates you can display any data from the related object in any format you wish.. If Relatedtotype =Account you can display any field of the Account and you can also display the related lists of Account .. For ex you can create a datatable with the value="{!Relatedto.Contacts}"... Similarly you can display other related lists...

You cannot use the <apex:page> tag inside your Email template.. Having said this you cannot use pageblock, pageblocksection or any other tag which is a child of the <apex:page> tag.. You can use outputfield,outputtext,datatable etc.. Do not attempt too much of styling and formatting for your email templates unless it is extremely necessary because you would'nt know how your email would be rendered by different providers.. Always use simple and standard formatting so that your email displays uniformly across all providers...