Add more terms to the glossary (prebid#5000)

* Add more terms to the glossary

* wordsmithing

---------

Co-authored-by: bretg <bgorsline@gmail.com>

Author: muuki88

overview/glossary.md

@@ -27,9 +27,35 @@ implementation) to the AdUnit, (the sellable item).
 ## AdUnit
 
 `Ad servers such as Google Ad Manager` - the AdUnit is the thing that business people care about. It's an
-inventory concept targeted by line items.
+inventory concept targeted by [line items](#line-items).
 
-[`Prebid Adunit`](/dev-docs/adunit-reference.html) - Prebid adds another layer to the ad server AdUnit: which header bidders should participate in the auction and their parameters.
+[Prebid Adunit] - Prebid adds another layer to the ad server AdUnit: which header bidders should participate in the auction and their parameters.
+
+## Placement
+
+A 'placement' is bidder term that's similar to [AdUnit](#adunit) -- it helps the bidder link the AdUnit to details about the header bidding auction that have been entered into their systems.
+Some bidders may use a combination of multiple parameters like zoneId, formatId, siteId or something else, but the concept is similar.
+
+{: .alert.alert-warning :}
+A notable exception is Google Ad Manager. [Google Ad Manager placements](https://support.google.com/admanager/answer/177397?hl=en) are a _group of [ad units](#adunit)_!
+
+## Line Items
+
+A 'Line Item' is an ad server concept that contains information about how a specific ad is intended to serve to a website or app. Line items are added to containers called 'Orders' and carry details like:
+
+- The number of times the advertiser or buyer wants an ad creative to serve
+- The cost negotiated for the campaign
+- Start and end times of the campaign
+- Targeting that allows the advertiser or buyer to reach its intended audience or demographic
+
+(Adapted from the [Google Ad Manager Line Item documentation](https://support.google.com/admanager/answer/9405477?hl=en))
+
+Integrating Prebid bids with other ad sources requires a [special line item setup](/adops/adops-planning-guide.html) if you aren't using an ad server that supports Prebid natively, which most don't.
+The [Prebid Line Item Manager] can help set up GAM line items. GAM 360 has the beginnings of a [native Prebid integration](https://support.google.com/admanager/answer/12273163?hl=en).
+
+## Key Values
+
+A generic way to target [line items](#line-items) in your ad manager. Prebid makes heavy use of key values to indicate the winning bidder and the winning bid price.
 
 ## AMP
 
@@ -39,10 +65,14 @@ also limits the scripts that can be run on the page. See [Prebid's AMP page](/fo
 ## Config ID
 
 `Prebid SDK` - when coding the mobile app for the technical implmentation of the adslot, the SDK function call
-refers to this ID that refers to an impression-level stored request on Prebid Server. This server-side stored
+refers to this ID that refers to an impression-level stored request on [Prebid Server]. This server-side stored
 request contains the bidders and parameters that can participate in the auction for that adslot.
 
-## MediaType or Media Format
+## Ad Formats
+
+This section contains terms around ad formats and their respective configuration.
+
+### MediaType or Media Format
 
 `banner` - also called 'display', these ads are generally simple rectangular images.
 
@@ -51,22 +81,26 @@ can be formatted to smoothly fit into the content.
 
 `video` - see Player-Initiated Video and Page-Initiated Video
 
-## Multiformat AdUnit
+### Multiformat AdUnit
 
 An AdUnit that can display creatives of multiple media formats: banner, native, and/or video.
 
-## Page-Initated Video
+### Page-Initated Video
 
 Also called 'outstream', these are AdUnits that work in Prebid.js just like banners, but are displayed with
 a special 'renderer' to play a video ad instead of a static image.
 
-## Player-Initiated Video
+### Player-Initiated Video
 
 Once called 'instream', these are AdUnits that live within a video player.
 
+### Interstitial
+
+Interstitial ads are full-screen ads that cover most of the visible UI. They're typically displayed at during navigation, e.g. clicking a link or going to the next level in a game.
+
 ## Prebid Mobile
 
-Prebid SDK and Prebid Server working together to provide header bidding for mobile apps.
+Prebid SDK and [Prebid Server] working together to provide header bidding for mobile apps.
 
 ## Renderer
 
@@ -83,3 +117,20 @@ associated with a particular adslot. There's a string ID for this block of JSON
 price granularity, targeting parameters, etc. There's a string ID for this block of JSON that can be used by the Prebid SDK or Prebid.js. In Prebid SDK, the "account ID" field in the init function currently doubles as the ID of the top-level stored request.
 
 `AMP stored request` - a block of JSON available to Prebid Server that stores both global and impression-level parameters. There's a string ID for this block of JSON that can be used by AMP RTC calls.
+
+## Server Side Bidding
+
+The header-bidding auction takes place on a server. Notable server side bidding systems are
+
+- [Prebid Server]
+- [Amazon TAM](https://aps.amazon.com/aps/transparent-ad-marketplace/)
+- [Google OpenBidding](https://support.google.com/admanager/answer/7128453?hl=en) formerly known as Exchange Bidding.
+
+## Client Side Bidding
+
+The auction takes place on the client device of the user.
+Prebid.js is a client side bidding solution.
+
+[Prebid Server]: /prebid-server/overview/prebid-server-overview.html
+[Prebid Adunit]: /dev-docs/adunit-reference.html
+[Prebid Line Item Manager]: /tools/line-item-manager.html