Electricity causes two problems. Burns when the current heats up your flesh, and stopping your heart.
For current to flow in your body, your body’s resistance needs to be low. So if your skin is wet then you are in trouble – if it’s dry you might fare better. If it’s hot weather, you’re most likely sweating a bit more so your skin is wetter and has lower resistance.
If the current flows through your chest, then it could stop your heart. Only needs something like 120mA or something which is not much at all – the kind of current that you might use for charging a mobile or iPod or something. It has to flow across your chest tho. If you grab a live and neutral wire in one hand, it’ll flow from the live to the neutral through your hand, and not kill you. It may burn your fingers depending on how damp your skin is. If you grab one wire in each hand, the current would flow up one arm, across your chest and down the other – game over (likely).
Incidentally, some people have drier skin than others and are less susceptible to being badly electrocuted IIRC.
Actually there is another way it can kill you – electricity causes muscles to contract so even if the current doesn’t flow through your chest it can cause your legs or body to spasm and you fling yourself off a pylon or across a room to smack your head on something.
People survive lightening strikes because they are out in the rain, and the current flows through their wet clothes or over their skin, causing severe burns but not stopping their heart.