Replace Timing Belt or Chain
Automobile engines use a timing belt or a timing chain to time the engines camshaft to the crankshaft. We are timing the piston travel to the opening and closing of the engines valves or (engine valve timing). This is important and must be done correctly, especially in interference engines.
In interference engines the valves travel deeper into the cylinder than non-interference engines. 
If these two shafts are out of time with each other the valve will strike the top of the piston resulting in bent valves, holes in the pistons, and possible cylinder damage. This is why we always recommend upon completion turning the crankshaft twice before attempting to start the engine. If you suddenly feel the crankshaft stop this is likely due to incorrect timing.
In a four stroke engine the crankshaft turns twice to every turn of the camshaft. We want the intake valve to open with the exact timing with the piston traveling down on its intake stroke. We also want the exhaust valve to open when the piston is traveling upwards on its exhaust stroke. It's also vital for compression to have both of these valves closed while the piston is on its compression stroke.
To remove and install a timing belt or chain on an engine you'll need to find the correct timing marks. Some engines have the camshaft located inside of the engine block like the famous Chevy 350 ci. Others especially later model vehicles have their camshafts located over the cylinder head. These camshaft marks must line up with their corresponding marks before removing any timing components. The same is true with the crankshaft sprocket (2:1 ratio).
Engines with in-block camshafts will typically use a timing chain. It will incorporate two sprockets the larger being the camshaft. 
Remember the camshaft spins at half the speed of the crankshaft so the camshaft sprocket will have twice as many teeth as the crankshafts. There will also be two marks to note: one on the crankshaft sprocket and one on the camshaft.
These cam in-block engines will usually have a dot or mark on the crankshaft sprocket that lines up at twelve o'clock at TDC (top dead center) and one on the camshaft that lines up at six o'clock. If there are no marks it is important that you make your own. You want to the engine set to TDC before removing the chain (marks lined up) so you can put the new chain and sprockets on exactly as the old ones came off.
Top dead center is the point in an engine when the #1 cylinder is on its compression stroke at the very top of its piston travel. This is usually marked with a line on an engines harmonic balancer that will line up with a zero on the timing cover. Another way to tell is by locating the #1 cylinder and removing the spark plug. While manually turning the crankshaft, there will be air forced through the spark plug hole as the piston travels upwards. This is because on the compression stroke both the intake and exhaust valves are closed.
After aligning the crankshaft with the camshaft you will typically remove the bolts fastening the sprockets to the shafts. Then remove the old chain sprocket assembly and replace them with the new ones and simply torque the bolt to specifications. Remember to put them in exactly the same position as the old ones and spin the crankshaft twice to assure no valve contact before reassembling engine.
Timing belts are typically installed on overhead engine and protected by seals from any oil exposure. They unlike the timing chain use the same sprockets over again.
They line up with marks that coincide with top dead center like a timing chain and are more prominent in import vehicles. It is super important that you use the correct marks when timing your engine as damage may occur: remember to always spin twice. Timing belts are changed more often, usually every 60 or 70 thousand miles.
The best way to tell if an engine is out of valve timing is by checking the compression. This is because when the valves are out of time the valve is likely to be open during the compression stroke causing the cylinder compression to escape through the open intake and exhaust valves. Most compression tools are inexpensive and simply screw into the spark plug hole. Compression is increased while the engine is turned a specified number of times. Check with manufacturer specifications and note and difference between cylinders.