Other variants:
Data: the cellular internet service, 4G, 3G, EDGE/GPRS, etc
SD Card: external storage card which can be inserted into the device to increase storage, normally only good for MP3s/pictures/videos/downloads, can move some, but not all apps, while most still use up the limited internal storage on the device, unless you're rooted and have an SD-EXT partition on the card which makes the internal storage as high as the card's capacity
Cache: parts of apps which are recently used and kept in memory to be quickly loaded later, can make some fooled into thinking their Android device is low on RAM, when in reality it's doing what it was designed to do; also refers to temporary storage for apps to use (saved data such as Play Store's home screen pictures, your Gmail e-mails, stored text messages, etc)
It is normally a good practice to regularly clear the app cache, or use an app designed to do it on a schedule. it is not a good idea to use a task killer, though.