Whats the diference between zener and normal diode in this circuit

Both are essentially the same in forward bias (as in your circuit). The difference lies in the reverse bias region, also called avalanche region, corresponds to the part of the I-V curve of the diode in the reverse region where the voltage is constant irrespective of the current. For normal rectifier diodes this voltage is typically high (up to 200v) and will be destroyed if operated in this region. For zener diodes this voltage is much less (lower than 10V, called zener voltage), zener diodes can be operated in the avalanche (zener) region, and are used to produce reference voltages for various applications.

For your circuit a normal rectifier diode is enough. The solar panel will charge the battery until it's voltage reaches 6V - 0.7V = 5.3V, after which the diode will be reverse biased and will stop conducting.

Thank you very much

http://everycircuit.com/circuit/5015565154648064