can you play Steam games though a sd card/ hard drive?

Steam options, download, button download directory, add the directory of the SD card or external hard drive.
Move the game installation directories to there. Maybe download a game first there to get the main directories (Steamapps, common, downloading etc). To move already installed games you can find their directory by right clicking on a game in library, manage, local files.
Saves are usually either in cloud or in a different directory in C:/ (user specific), usually you don't need to move them.

I never used external devices, so I can't share experience though.

I don't see why that would not be possible. If it's a device that your OS recognises there should be no problems.
Here's a slightly outdated guide from the Steam FAQ but it should be a similar process.

Yep, I can confirm it works out fine (as long as you get a decent SD Card). Here is a short video showing how to add a new "library" to Steam.

https://imgur.com/vhyt44t

P.S. Sorry for the missing mouse pointer, my screen capture got messed up probably. Also, sorry for the video quality, imgur compressed the hell out of it.

and run the games from them instead of the main laptop hard drive - is this possible?

I have Windows and Steam on my drive C, and use my other local drive (D) and external (F) as Steam games' folders, it's entirely doable

As long as you're not expecting to use the drive on multiple computers, it should be fine.
A lot of SD Cards have crappy speeds, so I would generally recommend an external SSD instead (they're getting pretty cheap now), or a grunty hard drive.

You can even move such storage between computers: when adding a library folder Steam recognizes the games it already contains. This also means that if you forget inserting the card into your laptop prior to starting Steam, you can just add it later.

As stated by others, yes, it´s absolutely possible and even transferring existing games to the new storage is relatively easy.
Once you created a new folder/path you´ll find the option under each games properties in your library. Loading times will depend very much on device speed and the game tho..

On a sidenote - don´t mess with your Saves!
Game-Saves in my experience are somewhere close to mystical rocket science and best left where Steam/each game wants them to be. To be safe I´d recommend using Steam´s Cloud service, if you can. :D

Yes its possible! I have two SSD drives on my laptop, you can choose to have the steam directory on the second drive, leaving the main drive mainly to the Operating System.

My advice is don't move games to the second directory if the game doesn't use steam cloud. Lost of couple of saves this way.
I would also advice to use the second directory to install the "new" games and over time delete the games from the first one.

It's certainly possible but not ideal. SD cards aren't meant for permanent read/write IO. In theory you can even run your OS from SD card but for how long is open for debate.

Speed might be the only issue?

External drives are fine. But if you're gonna play a demanding, large open world or competitive multiplayer game you want it on an internal HDD or SSD. For example when I play large open world games like Conan or Ark on and external HDD I notice stutters.

I even have games installed on a shared network drive, it works fine for 95% of games you won't even notice. Only the extremely disk I/O intensive games.

Some DRM/anticheat will not let you run a game from a removable or network drive. But those games are pretty rare.

does anyone have any subjections for a good ssd drive?