The cylinder head is comprised of a series of passages that carry the air and fuel combination needed for combustion to the cylinders; the liquid  coolant, needed to keep the engine cool; and harmful fumes (produced in the engine in the process of combustion) from the exhaust  valves to the exhaust manifold and away from the vehicle. An integral part of the engine's timing system, the camshaft works to manage this process by making sure that the intake and exhaust valves on the cylinder head are opening and closing at exactly the right time. The engine needs the correct amount of air and fuel flowing through it in order for the fuel to burn in the cylinders and for the engine to ignite or spark at the right time. The outer surface of the camshaft has protruding curves or lobes around it (one for each of the valves) which engage with, essentially lifting, the valves as the camshaft turns, opening and closing the intake and exhaust valves. valve guides work in conjunction with the valve spring to accurately engage/make contact with the intake and exhaust valves positioned in the cylinder head. 

valve guides are a circular section of metal, and fixed into the cylinder head. There's one valve guide per poppet valve. Specific materials used will vary, but usually valve guides will be manufactured from bronze or steel. 

The intake and exhaust valves, commonly referred to as poppet valves, sit in the valve seat when the valves are closed. The valve guides function to ensure contact can be made correctly between the valve and the valve seat. If the positioning between the valve and the valve seat is incorrect the space between the valve guide and the stem of the poppet valve will be incorrect. If there is too much space, the valve might not engage properly with the valve seat and if there's too little space then the valves could stick fast, which leads to the engine using far too much oil; both outcomes adversely affecting engine components and performance. The valve guides assist in this process by guiding the valves, in conjunction with the valve spring, to make precise contact with the valve seat when the valve is closed. The valve guides also function as heat conductors, removing the heat (produced by the combustion process) via the exhaust valve through into the cylinder head where it joins the cooling process.  

Subjected to high temperatures and friction, eventually the valve guides will suffer from wear and tear and will need to be replaced. When replacing this part, choosing to buy a product that is compatible with your car is important. The valve guides are designed specifically for the make and model of your vehicle, giving you a component that will install properly, work well and ultimately last longer.