Add RDD/DataFrame clarification

website/www/site/content/en/get-started/from-spark.md

@@ -2,14 +2,14 @@
 title: "Getting started from Apache Spark"
 ---
 <!--
-Licensed under the Apache License, Version 2.0 (the "License"); 
+Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
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 http://www.apache.org/licenses/LICENSE-2.0
 
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+distributed under the License is distributed on an "AS IS" BASIS,
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@@ -23,7 +23,11 @@ If you already know [_Apache Spark_](http://spark.apache.org/),
 learning _Apache Beam_ is easy.
 The Beam and Spark APIs are similar, so you already know the basic concepts.
 
-A collection of elements in Spark is called a _Resilient Distributed Dataset_ (RDD),
+Spark stores data _Spark DataFrames_ for structured data,
+and in _Resilient Distributed Datasets_ (RDD) for unstructured data.
+We are using RDDs for this guide.
+
+A _Spark RDD_ represents a collection of elements,
 while in Beam it's called a _Parallel Collection_ (PCollection).
 A PCollection in Beam does _not_ have any ordering guarantees.