scaffold explanations

Author: reint-fischer

docs/getting_started/explanation_concepts.md

@@ -5,6 +5,37 @@ kernelspec:
 ---
 # Parcels concepts
 
-The flexibility of Parcels enables a wide range of applications and the building of complex simulations. In order to help structure your code, this tutorial describes the structure that a Parcels script uses.
+Parcels is a set of Python classes and methods to create particle tracking simulations. Here, we will explain the basic concepts defined by the most important classes and functions. This overview can be useful to start understanding the names for different components we use in Parcels, and to structure and make appropriate use of the code in a simulation script.
 
-Code that uses Parcels is generally built up from four different components:
+A Parcels simulation is generally built up from four different components:
+1. [**FieldSet**](#1.-FieldSet). The input dataset of gridded fields (e.g. ocean current velocity, temperature) in which virtual particles are defined.
+2. [**ParticleSet**](#2.-ParticleSet). The dataset of virtual particles. These always contain time, z, lat, and lon, for which initial values must be defined, and may contain other variables.
+3. [**Kernels**](#3.-Kernels). Kernels perform some specific operation on the particles every time step (e.g. interpolate the temperature from the temperature field to the particle location).
+4. [**Execute**](#4.-Execution). Execute the simulation. The core method which integrates the operations defined in Kernels for a given time and timestep, and writes output to a ParticleFile.
+
+We discuss each component in more detail below and link to more detailed [how-to guides](../user_guide/index.md) and the full list of classes and methods in the [API reference](../reference.md). If you want to learn by doing, check out the [quickstart tutorial](./tutorial_quickstart.md) to start creating your first Parcels simulation. 
+
+![png](images/parcels_user_diagram.png)
+
+## 1. FieldSet
+
+Parcels provides a framework to simulate particles **within a set of fields**, such as flow velocities and temperature. To start a parcels simulation we must define this dataset with the `FieldSet` class.
+
+The input dataset from which to create a `FieldSet` can be an `xarray.Dataset` with output from a hydrodynamic model or reanalysis.
+
+### Grid
+read more about [grids](../user_guide/explanations/explanation_grids.md)
+
+### Interpolation
+read more about [interpolation](../user_guide/explanations/explanation_interpolation.md)
+
+## 2. ParticleSet
+
+
+## 3. Kernels
+read more about [Kernels](../user_guide/explanations/explanation_kernelloop.md)
+
+## 4. Execution
+
+### Output
+ParticleFile