Will speakers work with any receiver?

In general yes, speakers will work with any modern receiver. From the standpoint that the receiver is driving current to the speaker and moving the driver, all speakers will work as long as they are designed for input via speaker terminals.

Things to consider:

1. The speaker should be of appropriate ohm's to match the output of the receiver. This is generally 4-8 ohms.
2. The speaker should not have it's own amplifier built in. This could damage both the speaker and receiver.
3. The speaker should be of sufficient size depending on the amount of watts you're driving it with. Don't try and drive a 2.5" woofer with a 135 watt receiver, or at least don't try and turn it up to loudly.

Overall you should match the quality of the speaker with the quality level of the amplifier or receiver you're using. This has less to do with whether or not it will work and everything to do with the quality of the output you'll experience. The sound you eventually hear is only as good as the weakest component in your chain. In other words, using a $1,000 receiver with a pair of $50 speakers won't give you as good of results than if you purchased a $500 receiver and a pair of $500 speakers. 

Are speakers Analog or Digital

Speakers by their physical nature are inherently analog. They are a physical device affecting physical space. 

Then why are some speakers sold as "digital ready"?

Because, marketing. Speakers work when current, a small amount, is sent through the speaker wire to the electric current flows through the coil, it becomes a temporary electromagnet. The permanent magnet on the speaker is attracted and repelled which moves the speaker cone, which moves the air to create sound.  

Are some speakers better at working with digital sources than others? The short answer is no because the source has nothing to do it the speakers function. 

Does a digital source have an affect on the sound?

Yes, what will have an affect on the sound of a digital source is the type of DAC (Digital Audio Converter) that is in your amplifier.