Describe in-band and out-of-band terms (#714)

* Describe in-band and out-of-band terms

Signed-off-by: Yuri Shkuro <ys@uber.com>

* move anchors

Signed-off-by: Yuri Shkuro <ys@uber.com>

* Update specification/glossary.md

Co-authored-by: Christian Neumüller <christian+github@neumueller.me>

* Fix language

* Move to the top

Signed-off-by: Yuri Shkuro <ys@uber.com>

* Fix anchors

* Re-add anchor

Signed-off-by: Yuri Shkuro <ys@uber.com>

* fix link

Co-authored-by: Christian Neumüller <christian+github@neumueller.me>

specification/glossary.md

@@ -5,32 +5,48 @@ specification.
 
 ## Common
 
+<a name="in-band"></a>
+<a name="out-of-band"></a>
+### In-band and Out-of-band Data
+
+> In telecommunications, **in-band signaling** is the sending of control information within the same band or channel used for data such as voice or video. This is in contrast to **out-of-band signaling** which is sent over a different channel, or even over a separate network ([Wikipedia](https://en.wikipedia.org/wiki/In-band_signaling)).
+
+In OpenTelemetry we refer to **in-band data** as data that is passed
+between components of a distributed system as part of business messages,
+for example, when trace or correlation contexts are included
+in the HTTP requests in the form of HTTP headers.
+Such data usually does not contain the telemetry,
+but is used to correlate and join the telemetry produced by various components.
+The telemetry itself is referred to as **out-of-band data**:
+it is transmitted from applications via dedicated messages,
+usually asynchronously by background routines
+rather than from the critical path of the business logic.
+Metrics, logs, and traces exported to telemetry backends are examples of out-of-band data.
+
+<a name="telemetry_sdk"></a>
 ### Telemetry SDK
 
 Denotes the library that implements the *OpenTelemetry API*.
 
 See [Library Guidelines](library-guidelines.md#sdk-implementation) and
-[Library resource semantic conventions](resource/semantic_conventions/README.md#telemetry-sdk)
-
-<a name="telemetry_sdk"></a>
+[Library resource semantic conventions](resource/semantic_conventions/README.md#telemetry-sdk).
 
+<a name="exporter_library"></a>
 ### Exporter Library
 
-Libraries which are compatible with the [Telemetry SDK](glossary.md#telemetry-sdk) and provide functionality to emit telemetry to consumers.
-
-<a name="exporter_library"></a>
+Libraries which are compatible with the [Telemetry SDK](#telemetry-sdk) and provide functionality to emit telemetry to consumers.
 
 ### Instrumented Library
 
 Denotes the library for which the telemetry signals (traces, metrics, logs) are gathered.
 
 The calls to the OpenTelemetry API can be done either by the Instrumented Library itself,
-or by another [Instrumenting Library](#instrumenting_library).
+or by another [Instrumentation Library](#instrumentation-library).
 
 Example: `org.mongodb.client`.
 
 <a name="instrumenting_library"></a>
-
+<a name="instrumentation_library"></a>
 ### Instrumentation Library
 
 Denotes the library that provides the instrumentation for a given [Instrumented Library](#instrumented-library).
@@ -41,14 +57,15 @@ See [Overview](overview.md#instrumentation-libraries) for a more detailed defini
 
 Example: `io.opentelemetry.contrib.mongodb`.
 
-Synonyms: *Instrumenting Library*
-
-<a name="instrumentation_library"></a>
+Synonyms: *Instrumenting Library*.
 
+<a name="tracer-name"></a>
+<a name="meter-name"></a>
 ### Tracer Name / Meter Name
 
 This refers to the `name` and (optional) `version` arguments specified when
-creating a new `Tracer` or `Meter` (see [Obtaining a Tracer](trace/api.md#obtaining-a-tracer)/[Obtaining a Meter](metrics/api.md#meter-interface)). It identifies the [Instrumenting Library](#instrumenting_library).
+creating a new `Tracer` or `Meter` (see [Obtaining a Tracer](trace/api.md#obtaining-a-tracer)/[Obtaining a Meter](metrics/api.md#meter-interface)).
+The name/version pair identifies the [Instrumentation Library](#instrumentation-library).
 
 ## Logs
 
@@ -58,7 +75,7 @@ A recording of an event. Typically the record includes a timestamp indicating
 when the event happened as well as other data that describes what happened,
 where it happened, etc.
 
-Also known as Log Entry.
+Synonyms: *Log Entry*.
 
 ### Log