Normally (excluding stuff like intelligent dimmers), the switches are just switches.
What american electricians reffer to as a "3 way switch" and british electricians refer to as a "2 way switch"* is what electronics guys would call a SPDT switch. It has three terminals, one of which is a "common" which depending on the position of the switch connects to one of the other two terminals.
There are two maining wiring arrangements that can be used to control a light from multiple locations.
The first system and the easiest to understand is the system described in Dave's and Harper's answers. The permanent live is fed into the common terminal of one switch, the live to the light is fed. from the common terminal of the other switch. The other terminals of the switches are connected together by the "travellers". If the two switches are switched to the same traveller the light is on, if they are switched to different travellers the light is off.
The second system, which is the norm in the UK nowadays and which americans call "calafornia 3 way" or "coast 3 way", is to connect all three terminals of the switches together. Permanent live is taken from one non-common terminal and switched live is taken from the other. If both switches are switched to the permanent live line or both are switched to the switched-live line then the light is off. If one switch is switched to permanent live and the other to switched live then the light is off.
Which system is more convinient depends a lot on the design of your wiring accessories, if your accessories can only take one wire per terminal (which I understand is the norm in america), then the first system means less splicing. On the other hand if your accessories can take multiple wires per terminal (as is the norm in the UK) then the second system can avoid the need to splice.
To expand to more than two locations we use what American electricians call a Four way switch and british electricians call an intermediate switch. This swaps the travellers over and can be used to extend either of the above two systems.
There is also a third system, known by american as "Carter 3 way" but it is dangerous and should not be used. In this system the switches are used to switch between live and neutral. If the light gets two lives or two neutrals it is off, if it gets one of each it is on. This arrangement has several safety problems. Firstly it can leave the light "off but live". Secondly depending on the switch design it can lead to arcs between live and neutral. Thirdly in the case of ES lampholders it can leave the shell of the lamp live.
* Americans use the term "two way switch" to reffer to a simple on-off switch.