Everyone has devices around them these days, that need or have an option of a memory card. Choosing the right memory card or SD(Secured Digital) card for your device is a hassle free task if you take into consideration certain credentials. But would surely make a mess if you select a wrong one. The selection of a right memory card depends on the class, capacity and size of the memory card.

Class :
Memory cards are differentiated on the basis of speed class they belong to. Speed class is a measure used by the makers for measuring the speed of a memory card. They appear on all the SD cards and serve as a guideline for the buyer to find the best one for his requirements. The higher the class rating, better the reading and writing speed you get. For eg. Out of the 4 classes of SD cards, Class 10 is the fastest among all and Class 2 is the slowest. Class 10 is the fastest, hence best for Full HD video recording and clickng and saving photos in RAW format whereas Class 2 only supports SD quality recording. Classes 4 and 6 would perform decent in case of HD video recording. You might get class 10 as the costliest among the above 4 and class 2 being the cheapest.
Further there are 2 more classes, i.e UHS (Ultra High Speed) 1 and 3 of memory cards which are professionally used. These can be expensive but provide the ultimate performance.

Capacity :
Capacity here stands for storage capacity of SD ccards. SD cards are available in a variety of storage capacities.
SDSC (Secured Digital Standard Capacity) category includes SD cards ranging from 1MB to 2GB.
SDHC (Secured Digital High Capacity) category includes SD cards ranging from 2GB to 32GB. These are the most supported nowadays.
SDXC (Secured Digital Extended Capacity) category includes SD cards ranging from 32GB to 2TB. These are the newest. Check if your device supports it before you buy.

Size :
There are SD cards, mini SDs and micro SDs. The SD cards are of the standard size.
The mini SD cards are smaller than tha SD cards.
The micro SD cards are the smallest among all the SD cards.
Check the size that your device supports before buying one. You can however use a smaller card in the SD card slot using an adaptor which is nowadays generally shipped along with the micro SD cards.

While buying a memory card you will have to consider all the above factors, but along with that you will also have to check if your device supports them, or else you would not be able to derive the actual performance from the particular SD card or you may even not be able to use it even if it is of the highest specifications.

Below are some best buy links if you want to buy a memory card :

Sandisk Class 10 16GB Ultra Micro SD UHS-1 Memory Card

Sandisk Class 10 32GB Micro SDHC Memory Card

Sandisk Class 10 64GB Micro SDXC Memory Card