The Thailand Climate will vary a bit depending on the area or region you're in. The South gets monsoons from about May to October and that creates cool winds that stir up the coastal waters.
The Thailand climate far North is quite a bit different from the deep South and even a little different from the central region. The weather up North is a little more stable throughout the year because you're obviously further north and you've got the mountain areas that make the elevation higher. Both these factors make for decent year round weather. It's still hot though but not as bad as central and northeast.
Below, I give the seasons for the Central and Northeast; for the North just take off a few degrees and add more rain :-)
The climate in Bangkok Thailand and through the central region, including Isan in the Northeast, is pretty close to the same. Take a look at the following for an idea of the weather throughout the year.
This is the hot season. People not use to high temperatures should not come during this time.
Temps can go as high as 40 C (105 F) and at night you can really feel the humidity.
Bangkok Thailand weather seems hotter than the country side because you have the tuk-tuk and buses belching out smoke and the heat reflexing off the buildings, so I would recommend shorts in Bangkok whenever possible during this time of year if you're not use to the heat.
You'd be amazed at how many people wear jeans and slacks everyday when the weather in Bangkok is obviously hot! After awhile I too started wearing pants on a normal basis. I think your body gets better at regulating itself over a long period of time in the heat, but still it's not perfect.
The rainy season. I've always thought of the rainy season as not too bad. I've lived in Portland, Oregon and that's 6 months of pure cloudy rain. In Thailand, it really only rains for a couple of hours in the late afternoon or evening and then in the morning it's usually sunny.
Bangkok is prone to flooding during the rainy season as the city is at the mouth of a delta.
The cool dry season. Most days are sunny with a slight wind from the North. Temperatures only getting up to 30 to 35 C (85-95 F) with humidity at only 50%, heaven on Earth!
To be honest with you though, the Thailand climate is more humid than anything else during most of the year. I come from Texas and Texas summers ARE hotter than Thailand, believe me.