The Shared Ads section allows you to create text Ads that you can assign to multiple Ad Groups. You create a "list", give it a sensible name, and then create Ads just as you would within an Ad Group. Once you've created your Ads you then pick which Ad Groups you'd like to use these Ads and you're done!
Who would use it? Shared Ads are great when you've got multiple Campaigns sharing the same Keywords and Ads, for example when you've split Campaigns up for Location purposes but they're still effectively the same. Using Shared Ads in these Campaigns allows you to make changes only in one place and know for sure that they've been copied to all the various Campaigns.
Campaign Negative Keywords
Almost any Account is likely to have some Keywords that could be used as negatives in every single Campaign and probably a lot more negative Keywords that should be used in more than one Campaign and that's where Campaign Negative Keywords come in handy. Just like Shared Ads above, you create a list, give it a sensible name, fill it with negative Keywords and then you choose which Campaigns you want this list to apply to. Just like Shared Ads, this gives you one central place to maintain your negative Keywords and the reassurance they're being applied to all the right Campaigns.
Who would use it? Everyone, anyone!
Campaign Placement Exclusions
Campaign Placement Exclusions work in almost exactly the same way as Campaign Negative Keywords so I'm going to be real lazy and just say that all the text above applies in the same way, just pretend it says "Placement Exclusions" instead of "Negative Keywords".
Who would use it? Anyone with Placements they want to exclude from multiple Campaigns.
Bid Strategies allows you to define bidding strategies such as Enhanced CPC, CPA, Ad Position and Maximising for Clicks and then apply them to the Keyword or Group level. They're a great idea, although it's a real shame the ECPC and CPA strategies can only be applied at the Ad Group or Campaign level - I would have thought these would be ideal for applying to individual, high performance Keywords within Groups, but you'll just have to split them out on their own.
Who would use them? Probably for "mature" Accounts only where you're really getting into the tight detail of performance, but they're a useful tool to understand for anyone.
Shared Budgets allows you to specify a budget and then share it between two or more Campaigns. It's that simple. The Campaigns run and take their money from the shared pot.
Who'd use it? Shared Budgets are probably only useful in the short term since any established Campaign should really have a well-defined spending pattern you'll want to control individually but they are great for when you're splitting one Campaign into multiple copies - again, for example when you split by location or time or to separate Campaigns for any other reason. When you split a Campaign like this it can be difficult to estimate how the original budget needs to be split to fuel the new versions. With Shared Budgets you can just create a new Shared Budget that has the same value as the old single budget (or probably a little higher is a good idea) and then apply it to the new Campaigns. After a period you'll be able to see the trend for spending and revert the Campaigns to single budgets.
The Audiences section is where you define and manage your Remarketing lists and audiences. If you're not already using Remarketing and haven't applied your tag yet, do it now because you will want to use Remarketing in the future and the sooner you apply your tag, the bigger and more useful your list will be when you come to use it. Remarketing is a big topic in itself so you'll probably want to read through this next:
Remarketing Overview (AdWords Help Content)
Who'd use it? You may already be using Audiences in your Display or Search Ads but everyone should be at least trying Remarketing so look at this today!