I get distortions or inaccurate measurements on my map

The distortions and inaccurate measurements are related to geographic reference systems (CRS) that always have some distortions. Example of geographic CRS is WGS84 (EPSG:4326), which is the default CRS in QGIS and a very common CRS in open data and world data sets. Projected CRS minimise all these problems and should be used for analysis, but they are country or region specific. For example, in the UK it's the OSGB British National Grid (EPSG:27700) and in the Netherlands it's Amersfoort RD New (EPSG:28992).

EPSG codes are very useful to identify and select CRS because names can vary slightly. You can find everything about EPSG codes and the EPSG code for you country or region in http://epsg.org/, an then use it in QGIS.

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