Dedicated RAM is the amount of memory that is assigned for your virtual server; this is the fixed amount of memory that you get automatically during the purchasing of a hosting plan. Dedicated or guaranteed RAM is the most significant feature in hosting. In fact, how well (or bad) your hosting performs depends greatly on what amount of dedicated RAM you have on hand. It simply influences everything in your virtual server i.e. from speed to server efficiency, and uptime to power consumption. These features make it so important that in many cases, memory allocation actually serves as price determiner of the entire hosting package.
Another term you should take heed of is the ‘Burstable RAM', or in certain cases, the backup RAM. It is a temporary backup of resources over your standard memory quota. The burstable RAM can be offered to users of Virtual Private Servers. Each VPS has a guaranteed amount of RAM that it's able to use at most of the times. It is possible that the usage of web service may exceed the allotted quantity of guaranteed RAM available to us, this is when the backup RAM comes into play. It is provided to the user during times of maximum usage or traffic congestion. This RAM is not in use by any VPS on a server and can be allocated to any VPS that may need it as long as it is available. So as long as those resources are available, you as a user will be able to go over that limit of guaranteed RAM.
We can say burstable RAM is the RAM your VPS has the ability to use but it isn't guaranteed to. Basically it's for spikes in usage not consistent usage like guaranteed or dedicated RAM. Let take an example, if your VPS has 384 MB of guaranteed or dedicated RAM, and 1024 MB of burstable RAM. This means after you have used up your guaranteed RAM, there is still 640MB backup RAM available for burst usage. This is the only reason we always keep few extra memory with each server at all times as the burstable RAM.
The Burstable RAM is never guaranteed to be obtainable and should not be constantly depended upon for VPS operation. If your VPS constantly requires more resources than your package guarantees, they can quickly arrange an upgrade for you to a VPS hosting plan with much more guaranteed resources.