how to select a relay?

 Just wondering, How do you know what kind of relay to purchase for a particular use??  I was looking at them at radio shack, but there are so many different kinds.  One says -1 amp spdt micro relay Coil 12v dc, 37.5ma 32 ohms.  What is the difference some also say micro or plug in?? but they all look the same.  Is the ma the output of the signal???  Just trying to get some knowledge.

This should answer your question. https://www.the12volt.com/relays/relays.asp

I always swing by a local alarm shop and pick up supplies. sometimes they charge high or at least try to put most of the time its about right. They'll have what your looking for and the right kind. The down side is you have to listen to them brag and praise the brand they happen to be carrying that week and their personal feelings on EVERYTHING...

 Truth , I do not think the guys at the shop here know themselves.  I asked the other day, and the installer could not answer me.  When they installed my alarm, I asked where the valet switch was.  They  told me there is no switch.  That this would make it to easy for a thief to steeal.  Anyway a switch came in the box, but they kept it I guess??/  I think they try to makeit so , nobody can work on there products themselves.  My friend had bot a cd player and rear speakers.  When he came buy to show us.  There were no new speakers in the back shelf, just the old ones.  That speaks for itself.

that's true,  i quit taking my sh*t to installers because they also get your address when you buy stuff or get stuff put in your car.Factory security systems suck!

yah , I know some real good and honest installers.  I would, and have highly recommended.  Although the one that I had  install my last system does really s--k.