GeoJSON is a format for representing geographic data structures. As the name suggests it follows JSON (Javascript Object Notation) rules for writing these data structures, hence can be easily provided to browser applications for plotting different data sets. Each GeoJSON object may represent a geometry, a feature, or a collection of features.
GeoJSON geometry types can be Point, LineString, Polygon, MultiPoint, MultiLineString, MultiPolygon, and GeometryCollection. Features in GeoJSON contain a geometry object and additional properties(like: color, country, name etc.), and a feature collection represents a list of features.

These Geojson objects could be directly kept in a javascript object or provided to browser in a json file through Ajax request.
A typical Geojson object looks like:

{ 
 "type": "FeatureCollection",
 "features": [
    { "type": "Feature",
       "geometry": {"type": "Point", "coordinates": [78.9629, 20.5937]},
       "properties": {"Name": "India"}
    },
    { "type": "Feature",
       "geometry": {
      "type": "LineString",
      "coordinates": [
         [ 75.0585, 22.7761], [78.6840, 22.7761], [82.3974, 22.8369]
         ]    },
      "properties": { "name": "A_Line",
                      "color": "#227455"
                    }
    },
    { "type": "Feature",
       "geometry": {
       "type": "Polygon",
       "coordinates": [
                [   
                  [76.8164, 27.2936], [75.3222, 25.0597], [75.3222, 21.9430], 
                  [80.5957, 22.3500], [80.3320, 25.5972], [78.6621, 27.3717],
                  [76.8164,27.2936]
                ]
            ] },
       "properties": {
       "prop0": "A_Polygon",
       "prop1": {"this": "that"}
       }
     }
  ]
 }

There are various data sets of Geojson available online for plotting different countries, states etc.

It is also possible to create your custom Geojson data set through different online tools like geojson.io.
These tools can also be used to validate your Geojson data, but it would be good for you if they are used with only trivial/light data sets, as large data sets may result in browser paralysis.
In that case, you will have to develop and use some of the good open source frameworks available like Leaflet for data plotting and visualization.

For more info on Geojson follow.
You will be able to do some great things using Leaflet by going through upcoming articles on Leaflet.