Weather effects guitars generally upon two factors: temperature and humidity.

Ideal Conditions for a guitar are most importantly concerned with humidity and temperature. Ideal humidity is 40-50%, and ideal temperature is 70-75F (21-24C).

Heat will make everything in your guitar expand, the wood tends to bulge slightly, like how an old wooden door becomes harder to open in the summer time. Playing your guitar is extreme heat also causes your hands to sweat more which gets on the strings and causes them to corrode faster. Cold does exactly the opposite. All the wood shrinks, and sometimes once-tight fittings become loose. Also, if you pick up a cold guitar and tune it, it will quickly go out of tune as you play it because the heat of your hands on the strings, and the heat of your torso on the guitar body will warm it back up.

Extreme humid climates cause the wood in your guitar to soak up moisture, whereas the other extreme of dry climates will pull all moisture out, causing possible cracking or warping.

The good news is that 70-75 F is generally "room-temperature", so as long as you don't keep your guitar in your backyard you should be fine. Humidity, however, can be a bit trickier. Most people live in more extreme climates on either end of being very dry or very wet. To that end you may do well to purchase a guitar humidifier, which can be found at any local music dealer. This will help compensate for humidity in either needed direction.