Ram extension using microsd. Only suitable for low end phones.
They work just ok, as microsd cards are very much slower than the speed of ram. Class 10 or faster speed memory cards work good.
If phone gets slow, check free ram available. If it is less than 75-50 mbs available, then you may need to either extend it or close some apps.
Settings > application manager > running.
However, I prefer greenify, as it hibernates (closes) apps when they are not being used, hence freeing ram and conserving battery consumption.