The DHT range of sensors are very cheap sensors that provides both temperature and humidity in a digital form.
This means that actual numbers are provided for both temperature and humidity which is an advantage over other methods that include non-linear thermistors and capacitor based humidity sensors both of which require specific calibration and/or more advanced circuits.
However, the timing of the DHT communication is so short as to not be accessible by PICAXE. Others on the forum (such as Maguin Michel) have come up with a solution to allow DHT sensors to work with 64MHz PICAXE chips (i.e. most of the X2 range). This solution requires a 555 timer IC to help shape the transmission into a form 64MHz PICAXE can receive. I've taken this idea one step further and added in a second 555 and shaped the transmission into a form 32MHz PICAXE can receive. Although both the circuit and program are a little complicated, it now means that 32MHz PICAXE chips, such as 08M2, can access the cheap DHT sensor range.
I found this a great learning project as it highlights two implementaitons of the 555 timer, one to lengthen a pulse and one normally used for "missed pulse detection".