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+# Crash Course: containers
+
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+# Table of Contents
+
+* [Introduction](#introduction)
+* [Containers](#containers)
+  * [Dense map](#dense-map)
+  * [Dense set](#dense-set)
+
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+
+# Introduction
+
+The standard C++ library offers a wide range of containers and it's really
+difficult to do better (although it's very easy to do worse, as many examples
+available online demonstrate).<br/>
+`EnTT` doesn't try in any way to replace what is offered by the standard. Quite
+the opposite, given the widespread use that is made of standard containers.<br/>
+However, the library also tries to fill a gap in features and functionality by
+making available some containers initially developed for internal use.
+
+This section of the library is likely to grow larger over time. However, for the
+moment it's quite small and mainly aimed at satisfying some internal needs.<br/>
+For all containers made available, full test coverage and stability over time is
+guaranteed as usual.
+
+# Containers
+
+## Dense map
+
+The dense map made available in `EnTT` is a hash map that aims to return a
+packed array of elements, so as to reduce the number of jumps in memory during
+iterations.<br/>
+The implementation is based on _sparse sets_ and each bucket is identified by an
+implicit list within the packed array itself.
+
+The interface is very close to its counterpart in the standard library, that is,
+`std::unordered_map`.<br/>
+However, both local and non-local iterators returned by a dense map belong to
+the input iterator category although they respectively model the concepts of a
+_forward iterator_ type and a _random access iterator_ type.<br/>
+This is because they return a pair of references rather than a reference to a
+pair. In other words, dense maps return a so called _proxy iterator_ the value
+type of which is:
+
+* `std::pair<const Key &, Type &>` for non-const iterator types.
+* `std::pair<const Key &, const Type &>` for const iterator types.
+
+This is quite different from what any standard library map returns and should be
+taken into account when looking for a drop-in replacement.
+
+## Dense set
+
+The dense set made available in `EnTT` is a hash set that aims to return a
+packed array of elements, so as to reduce the number of jumps in memory during
+iterations.<br/>
+The implementation is based on _sparse sets_ and each bucket is identified by an
+implicit list within the packed array itself.
+
+The interface is in all respects similar to its counterpart in the standard
+library, that is, `std::unordered_set`.<br/>
+Therefore, there is no need to go into the API description.