guide: intro run-cache, change H3->H2 in Basic Concepts

and other copy edits

content/docs/user-guide/basic-concepts.md

@@ -4,7 +4,7 @@ DVC streamlines large data files and binary models into a single Git
 environment. This approach will not require storing binary files in your Git
 repository.
 
-### DVC Project
+## DVC Project
 
 Initialized by running `dvc init` in a directory, it will contain all the
 [DVC files and directories](/doc/user-guide/dvc-files-and-directories),
@@ -15,15 +15,15 @@ files referenced from special DVC files are also considered part of the project
 > `dvc destroy` can be used to remove all DVC-specific files from the directory,
 > in effect deleting the DVC project.
 
-### DVC repository
+## DVC repository
 
 <abbr>DVC project</abbr> initialized in a Git repository. This enables the
 versioning features of DVC (recommended). Files tracked by Git are considered
 part of the DVC project when referenced from special DVC files such as
 `dvc.lock`, for example source code that is used as a stage
 <abbr>dependency</abbr>.
 
-### Data Files
+## Data Files
 
 Large files (or directories) that are tracked and <abbr>cached</abbr> by DVC.
 Data files are too large to be added to a Git repository. DVC stores them on a
@@ -37,32 +37,32 @@ performance data, etc.
 
 > A.k.a. <abbr>data artifacts</abbr> and <abbr>outputs</abbr>
 
-### Workspace
+## Workspace
 
 It's comprised by the non-internal <abbr>project</abbr> files, as well as the
 currently present set of _data files_ and directories (see `dvc checkout`).
 Similar to the
 [working tree](https://git-scm.com/docs/gitglossary#def_working_tree) in Git.
 
-### DVC Cache
+## DVC Cache
 
 A DVC project's <abbr>cache</abbr> is an
 [internal directory](/doc/user-guide/dvc-files-and-directories#structure-of-cache-directory)
 used to store all data files outside of the Git repository. It's a local hard
 drive or external location. See `dvc cache dir`.
 
-### Remote Storage
+## Remote Storage
 
-Storage location external to the DVC project, which is used to backup all or
-parts of the <abbr>cache</abbr>. See `dvc remote` for more details.
+Storage location external to the DVC project, which is used to share and backup
+all or parts of the <abbr>cache</abbr>. See `dvc remote` for more details.
 
-### Processing Stage
+## Processing Stage
 
 An individual process that transforms a data input (<abbr>dependency</abbr>)
 into some result (usually a data <abbr>output</abbr>). DVC stages execute
 terminal commands to (re)generate their results.
 
-### Data Pipeline
+## Data Pipeline
 
 Dependency graph ([DAG](https://en.wikipedia.org/wiki/Directed_acyclic_graph)),
 or series of [data processing stages](#stage) to (re)produce certain results.
@@ -71,7 +71,7 @@ defined in special `dvc.yaml` files. Refer to `dvc dag` for more information.
 
 See [Data Pipelines](/doc/start/data-pipelines) for a hands-on explanation.
 
-### Reproducibility
+## Reproducibility
 
 Action to reproduce an experiment state. This regenerates output files (or
 directories) based on a set of input files and source code. This action usually
@@ -80,9 +80,7 @@ changes experiment state.
 > This is one of the biggest challenges in reusing, and hence managing ML
 > projects.
 
-## Advanced Concepts
-
-### Experiment
+## Experiment
 
 An attempt at a data science task. Each one can be performed in a separate Git
 branch or tag, and its states identified by different
@@ -93,7 +91,18 @@ experiment into the <abbr>repository</abbr> history.
 
 > See [Experiments](/doc/start/experiments) for a hands-on explanation.
 
-### Workflow
+## Run Cache
+
+DVC's run-cache is an automatic performance feature that stores both the context
+and results of past experiment runs. It's located in the `.dvc/cache/runs`
+directory.
+
+`dvc run` and `dvc repro` look in the run-cache first before executing any
+stages, to see if this exact same configuration has been run before (and if so
+use the cached results). The run-cache can be uploaded and downloaded to/from
+remote storage, along with the rest of the <abbr>cache</abbr>.
+
+## Workflow
 
 Set of experiments and relationships among them. Corresponds to the entire
 <abbr>project</abbr> and may contain several [data pipelines](#data-pipelines).