What a lot of bulldust ,
The current in your battery is totaly different to the 240 volt A/C in the house mains . Whatever you do dont play with the house power , that can kill you .
The car battery is D/C ( direct current ) and it will not kill you at 12 volts , altough it has heaps of amps .
If you happen to touch the + and - with your body nothing will happen you wont even know it ( unless your hands are wet ).
If you put the spanner or a wire across the terminals then you will have sparks , it can burn you ( the wire will melt , spanner will get very hot , red hot ) .
The reason you disconnect the - ve first is if you touch the cars body with the spanner you dont get fireworks . when the - is disconnected the dody doesn't have - so if you touch the spanner on to it when you undoing the + nothing will happen .
Take care with it and you will be o.k.