With capacitors, if you can't replace like-for-like insofar as values are concerned, it's best to go higher. Going lower isn't recommended. There are some good sites concerning capacitors that are readily found with Google.
Essentially, voltages are the maximum ratings for the capacitor - installing a 50v cap in a 12v circuit is fine (the capacitor doesn't have to work as hard if it's dealing with a lower voltage rating than it's built for), but installing a 5v cap in a 40v circuit will usually kill them.
The capacitance ratings are important as they dictate how much filtering or charge storing capacity is provided by the component. Any lower than that specified by the circuit may cause issues, such as lower average voltages or current ripples (in AC-DC filters, for example).
Next is temperature ratings. Most caps are rated for 85 C maximums, and they are the most common. However, some applications call for 105 C-rated caps, and I'm sure that some can go higher than that. If the capacitor is going to be used in a hot environment (such as the ones in my car's engine computer), it's best to use the higher temp type. Using an 85 C part where a 105 C part was installed will likely result in a shortened component lifespan.
The specifics for all of the above are generally listed on the component.
Final notes: Make sure to replace electrolytics with electrolytics (and ceramics with ceramics, etc, if they're bad), and to install an electrolytic capacitor in the correct orientation. Also, keep in mind that capacitors with higher values are larger than their lower-value counterparts, so you may not be physically able to install a 50v 47uf component where a 16v 33uf component existed previously. I used to know of a pretty good website where you can order almost any capacitors - I can dig it up if you want.
Essentially, voltages are the maximum ratings for the capacitor - installing a 50v cap in a 12v circuit is fine (the capacitor doesn't have to work as hard if it's dealing with a lower voltage rating than it's built for), but installing a 5v cap in a 40v circuit will usually kill them.
The capacitance ratings are important as they dictate how much filtering or charge storing capacity is provided by the component. Any lower than that specified by the circuit may cause issues, such as lower average voltages or current ripples (in AC-DC filters, for example).
Next is temperature ratings. Most caps are rated for 85 C maximums, and they are the most common. However, some applications call for 105 C-rated caps, and I'm sure that some can go higher than that. If the capacitor is going to be used in a hot environment (such as the ones in my car's engine computer), it's best to use the higher temp type. Using an 85 C part where a 105 C part was installed will likely result in a shortened component lifespan.
The specifics for all of the above are generally listed on the component.
Final notes: Make sure to replace electrolytics with electrolytics (and ceramics with ceramics, etc, if they're bad), and to install an electrolytic capacitor in the correct orientation. Also, keep in mind that capacitors with higher values are larger than their lower-value counterparts, so you may not be physically able to install a 50v 47uf component where a 16v 33uf component existed previously. I used to know of a pretty good website where you can order almost any capacitors - I can dig it up if you want.