Well, you should have the ssd set up as the primary drive for windows and anything that needs fast access like a big mmo game, and the HHD is used for general storage because it is not so fast. In short, EVERYTHING is stored on the hard drives. You do not need a SSD but it makes a big difference to system response times.
They store file information; permanent data like game files, word documents, etc.
The difference comes in with HOW they store this data. Both have advantages and disadvantages.
SSD = Solid State Drive
These are essentially big, glorified USB sticks, with better flash chips. Since they work on the basis of electrochemical cells, data retrieval is extremely fast and writes are generally very fast too. Unlike a HDD, they do not have to seek within the realm of physical, 3D space, in order to find an item. An HDD has to spin a platter to find each sector of data, which takes time. A solid state gets an address from the filesystem table, and goes straight to it.
However, they have two major cons:
- They reach density problems far faster, which means higher capacities have an increasingly higher monetary cost. This also makes them slower as they fill up; not massively, but it can be noticeable, especially if TRIM is not properly enabled.
- Flash cells are, in theory, far less durable than a hard disk platter, because they have to overwrite an entire cell to simply change a single bit. The process is write intensive, and so for what might be one write on an HDD, an SSD would experience the equivalent of, say, 4.
Good, reliable mechanical storage. It's a lot slower for anything except data in sequence (bit 1.1, 1.2, 1.3 vs. 1.1, 1.2, 2.1, 1.3) since like an old record player, it has to find the sector where a file is stored in physical space by spinning the platters until it has found it. Data is not always in adjacent sectors, so this slows down a PC. However, they have higher read/write tolerance than an SSD; this is weighed off against the cost of deteriorating moving parts.
Basically: SSD is great for speed and random-access (where memory locations are not adjacent) while a mechanical HDD is great for storage capacity and does well enough at sequential data read/write. SSDs are good for your OS partition, level-loading games, or anything else that pulls from random locations on disk. HDD is good for everything else.
As an example:
Perfect file for an HDD: Music. Since it is read and written in sequence, once it is found, it's easy to get the info.
Perfect file for SSD: OS. Since your computer will jump around finding the files it needs during boot, it's quicker for an SSD.
Note: Reliability on SSDs is really not an issue. Many will tolerate up to 700TB (absolute terms) of writes without failing. That's a lot.
Think of the two like a neighbourhood of houses, 1, 2, 3, 4, and 5 (side by side, in that order, along a street)
To load a music file, you need to get to houses 1, 2, 3, and 4.
an HDD will find house 1, then you walk to house 2, 3, and 4.
an SSD teleports you to house 1, then 2, then 3, and 4.
Since you only jump one house at a time, in order, you don't save much time teleporting vs. walking.
To load a random-access file (Houses 1, 3, 2, 4):
An HDD will find the first house, say, #1. Then you walk to house 3. Then you walk to house 2. Then you walk to house 4. You can see how that would be inefficient.
An SSD will start at house 1. Teleport you to house 3, then 2, then 4.
However, since you're not going in order here, the teleports save you the time covering ground you already have. It's far more efficient.