Arduino Sensors

[AHarris]

I am new to the idea of Arduino however from what I have seen it is a clear it is an extremelly powerful tool and one that can happily sit in any ROV and provide vital information that would otherwise be extremelly different.
I understand the idea of buying a the board and that you can add bits to it, then using a computer programme you can tell the Arduino what to look for and what to do when it has found it however I am not sure on the sort of things you would add.
From other examples I have noticed that they seem to be very simple speaker, LED type bits and bobs however if I wanted to record something like temperature, light levels and leak detection (that I understand is possible) what sort of sensors would I need?
Would I need to buy £50 or £60 ($100-$120) temperature probes or can I make it with of the shelf parts from the local hardware store?

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Update:
I realised I was looking at a laboratory grade sensor accurate to something like 1/100th of a degree.
I did however find the following website;
www.parallax.com

Would the sensors that they are supplying in their shop (example included below) be suitable for attaching to some of the pins on an Arduino control board providing the programme is correct?

http://www.parallax.com/Store/Sensors/T ... fault.aspx

[SoakedinVancouver]

Do a local search at your electronic suppliers. You are looking for thermistors. They are a temperature sensitive resistor, and therefore easy to physically interface to PIC's and/or Arduino's. Software-wise they are a little bit of a bother in some case because their resistance does not change linearly with temperature, but sometimes it is of no importance, depending on the temperature range you are looking for. Do some research on them, you'll understand what I am babbling about.