Hi Terri, as Chris says, you can play in offline mode. To access offline mode just click on the topmost link in the top left of the client where it says Steam. You'll get a drop down menu, with Go Offline... being the second option.
whether this will reduce the amount of resources used, I really don't know.
That said, depending on the game, some can be played directly from where it's installed within the Steam clients installed folders without Steam being on at all. Not all games can be played like this, but some can.
To find out where your game is installed right click on the game in your Steam library; then, from the drop down menu select Properties.
This opens a window see the image below:
Click on the local files in the left hand menu and you'll then see another window that looks like the one below.
From this select Browse. This then opens directly to the games open folder, showing you what's inside the folder e.g.
Make a note of the location of the game, or better still create a desktop shortcut by right clicking on the games executable. Turn off Steam, then see if you are able to play the game directly from the installed game via the shortcut you've just created.
Do this with every game you want to play without Steam. As I said above; not all games will work this way because they may be tied to the Steam client; if they are, you can still play them offline unless they are also linked to other clients such as many of the EA games that are linked to Origin, or games that were released by Ubisoft that are also linked to their client called uPlay.
for example, EA games recently released a whole bunch of their games that they had refused to allow on Steam for several years following a dispute of DLC. You can now buy those games on steam, but when you try to play them via the Steam client, you then have to boot up Origin, sign into your Origin account and then install it via Origin before you can play any of those games.
p.s.
If all your games are installed on the same drive, all you need to do after finding the first one is click on common that you can see at the top of the files listed in the game and it will step back to the folder where all of your games are installed to. e.g.