Updating urn.md doc
-# What is URN? [WIP]
+# What is URN?
+[URN] (Uniform Resource Name) is the chosen scheme of [URI] to uniquely define any resource in DataHub. It has a form as below:
+```
+urn:<<namespace>>:<<entity-type>>:<<content>>
+```
+Any [entity onboarding](../how/entity-onboarding.md) in [GMA](gma.md) starts with modelling an URN specific to that entity.
+You can check URN models for already availabile entities [here](../../li-utils/src/main/java/com/linkedin/common/urn).
+
+### Namespace
+All URNs available in DataHub are using `li` as their namespace.
+This can be easily changed to a different namespace for your organization if you fork DataHub.
+
+### Entity Type
+Entity type for URN is different than [entity](entity.md) in GMA context. This can be thought of as the object type of
+any resource for which you need unique identifier for its each instance. While you can create URNs for GMA entities such as
+[DatasetUrn] with entity type `dataset`, you can also define URN for data platforms, [DataPlatformUrn].
+
+### Content
+Content is the locally unique identifier part of an URN. It's unique for a specific entity type within a specific namespace.
+Content could contain a single field or multi-field (complex URN). Complex URNs could even contain another URN as one of their content fields.
+Example URNs with a single field content:
+
+```
+urn:li:dataPlatform:kafka
+urn:li:corpuser:jdoe
+```
+
+[DatasetUrn] is an example of a complex URN (multi-field content). It contains 3 fields: `platform`, `name` & `fabric`, respectively.
+Below are some examples:
+```
+urn:li:dataset:(urn:li:dataPlatform:kafka,PageViewEvent,PROD)
+urn:li:dataset:(urn:li:dataPlatform:hdfs,PageViewEvent,EI)
+```
+
+[URN]: https://en.wikipedia.org/wiki/Uniform_Resource_Name
+[URI]: https://en.wikipedia.org/wiki/Uniform_Resource_Identifier
+[DatasetUrn]: ../../li-utils/src/main/java/com/linkedin/common/urn/DatasetUrn.java
+[DataPlatformUrn]: ../../li-utils/src/main/java/com/linkedin/common/urn/DataPlatformUrn.java
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