Mirada? Cabeceo? What’s the difference?

In essence they are two very simple signals in the milonga:

MIRADA (the look) = showing interest to dance

CABECEO (the nod) = indicating acceptance

Yet even experienced tango dancers may find this respectful form of invitation challenging.  

For example, I’ve often heard:
“I was nodding at you, so why didn’t you dance with me?”

“He/She stares at me continuously even though I show no interest.”

“He/She is always in deep conversation with friends.  Later he/she complains that we didn’t dance.”

Well, let’s take a look at the basics:

MIRADA
Having selected a person you would like to dance with, you indicate your interest by looking at their eyes.  The look usually needs to be sustained for a few seconds - no more, so they understand that you would be interested in dancing with them.  A glance is not usually enough.  But it’s not a long stare either.

(If they’re not interested at the time, they won’t hold your gaze.  And nobody else will know!)

CABECEO

If the person does hold your gaze, they may accept by nodding their head, or by smiling, or by making some other positive gesture. You can then show your agreement with a similar gesture.  While maintaining eye-contact, the man makes his way to the woman’s table, who remains seated until he arrives in front of her (thus avoiding misunderstandings in a busy milonga).  He then leads her on to the dance floor - yes, he goes first.

Yes, it’s that simple! 
If you’re interested you need to show it through your body language. 
And you need to accept that everyone has a right to choose. In other words, the acceptance must be mutual, otherwise it’s not consent.

Of course, there are lots of finer points. To give yourself the best chance of success, I recommend taking a good look at Tango Mentor’s excellent article.
https://tangomentor.com/cabeceo-beginners-guide/