It is static electricity. Cars, themselves, can get charged with respect to "ground", though they will discharge reasonably quickly because the tyres are made electrically conductive to reduce this effect. From what you describe, it is more likely to be due to the clothes you are wearing and the material of the seat causing you to charge up. Touching the car causes the discharge (i.e. the spark). This will be worse if the car seats are cloth or plastic rather than leather. To avoid the problem, try keeping hold of a bit of metal of the car (like the door) as you get out. This way the discharge will occur gradually without the unpleasant spark.