Merge pull request #1910 from ipfs/add-mainnet

Add IPFS "Mainnet" to the glossary 

Credit to @bmann for coining the term.

docs/concepts/glossary.md

@@ -76,7 +76,7 @@ A Bootstrap Node is a trusted peer on the IPFS network through which an IPFS nod
 
 ### CAR
 
-The CAR (Content Addressable aRchives) is a format for serialized representation of any [IPLD](#ipld) [DAG](#dag). Typically in a file with a .car filename extension or a byte stream marked as [application/vnd.ipld.car](https://www.iana.org/assignments/media-types/application/vnd.ipld.car) media type. [More about CAR](https://ipld.io/specs/transport/car/)
+The CAR (Content Addressable aRchives) is a format for serialized representation of any [IPLD](#ipld) [DAG](#dag). The CAR format is a way of packaging up content-addressed data into archive files that can be easily stored and transferred. Typically in a file with a `.car` filename extension or a byte stream marked as [application/vnd.ipld.car](https://www.iana.org/assignments/media-types/application/vnd.ipld.car) media type. [More about CAR](https://ipld.io/specs/transport/car/)
 
 ### CAR v1
 
@@ -208,7 +208,7 @@ An experimental data store used when `--nocopy` is passed to `ipfs add`. It stor
 
 ### Gateway
 
-An IPFS Gateway acts as a bridge between traditional web browsers and IPFS. Through the gateway, users can browse files and websites stored in IPFS as if they were stored on a traditional web server. [More about Gateway](../concepts/ipfs-gateway.md) and [addressing IPFS on the web](../how-to/address-ipfs-on-web.md)
+An IPFS Gateway is an HTTP server that acts as a bridge between web browsers and IPFS. Through a gateway, users can browse files and websites stored in IPFS as if they were stored on a HTTP web server. Most commonly, an IPFS Gateway is an IPFS node that also exposes an HTTP IPFS Gateway endpoint. [More about Gateway](../concepts/ipfs-gateway.md) and [addressing IPFS on the web](../how-to/address-ipfs-on-web.md)
 
 ### Garbage Collection
 
@@ -298,6 +298,10 @@ In IPFS and [IPLD](#ipld), a _link_ usually means a pointer to some [CID](#cid).
 
 ## M
 
+### IPFS Mainnet
+
+IPFS Mainnet is a term used to describe the default or "main" public network that most IPFS implementations connect to by default. Most [IPFS implementations](https://specs.ipfs.tech/architecture/principles/#ipfs-implementation-requirements) were designed to work with Mainnet (but some can be configured instead to form a private swarm). Mainnet IPFS nodes typically join the [Amino DHT](#amino) for content routing with the help of the [Bootstrap nodes](#bootstrap-node), rely on [Bitswap](#bitswap) for data transfer, [UnixFS](#unixfs) for encoding files and directories, and typically expose an [IPFS Gateway](#gateway). This has mostly been assumed for the IPFS network. Nonetheless, IPFS Mainnet is a useful distinction in a world of many [IPFS implementations](../concepts/implementations.md) with varying degrees of interoperability.
+
 ### Merkle-DAG
 
 The Merkle-DAG is a computer science data structure used at the core of IPFS files/block storage. Merkle-DAGs create a hash to their content, known as a [Content Identifier](#cid). [More about Merkle-DAG](merkle-dag.md)