What is this type of SSD called?

Most likely this SSD is/was used as part of an ExpressCache setup. The idea is/was to offload certain caching operations of the HDD onto an SSD for speed. It may have been advertised as a "hybrid drive" or "solid-state cache" or similar. In my experience the benefit is rather marginal.

The SDD is probably only useful when running the proprietary caching software that came preinstalled on the machine. On my Y510p, this disk shows up in Windows as an OEM partition on Disk 0 in Disk Management and I can't, for example, format the volume to make it available to the filesystem for storage or as a boot partition. The software runs as "ExpressCache Service".

Replacing it with a higher capacity mSATA SSD might eke out some more performance if you can somehow make sure that the caching software recognizes it. I don't think you'll be able to use that slot to mount another storage device.* If you install a SATA SSD to replace the HDD, you might even consider disabling the caching software and removing this SSD, as there will be no benefit to having an SSD cache for an SSD.

*Edit: I searched around a bit and found a few stories (example) of successful use of the mSATA slot to mount a drive to be used for boot or storage. I would be careful, as this use was probably never tested or recommended by the manufacturer.

It's mSATA as seen here:

A search for the laptop manual, should have told you.

24GB is not huge, replace it if you must, but I would look for a new laptop.

it's mSATA, it might be faster and easier to google the part number (SMS151S3/24G) to get an answer for similar questions (that's how I found it and got this https://tech4drive.com/product/SMS151S3-24G) the laptop may not support a newer or bigger SSD, and it also may not be worth replacing anyway, there are likely other bottlenecks in your hardware, if you replace the regular 2.5" sata drive with a ssd it will be more likely compatible if you get a new computer