<p>I believe that Agile is nothing but another implementation of Spiral Model. I am a big supporter of Spiral (The spiral model is a software development process combining elements of both design and prototyping-in-stages, in an effort to combine advantages of top-down and bottom-up concepts) since its beginnings and have seen that lot of projects implement Spiral without knowing that they are operating in a Spiral world. Since the day Agile started gaining popularity the concept of spiral started getting overlooked a little bit. I am sure that for complex projects spiral is still the best alternative but I would like to get a better understanding of the similarities and differences between Agile and Spiral techniques.   Can anyone explain their differences/similarities?</p>

Agile
is
spiral.  Totally.  In part, the name was changed for marketing purposes.
The problem is that spiral tends to imply "big design up front" -- where you plan out many spirals, each in order of risk.  Spiral, however, isn't Agile -- it's just incremental execution in order of risk.
One big distinction that Agile adds is the "don't overplan things you can't know yet."
Agile
is
spiral, but you create detailed plans for just
one
increment at a time.
Agile adds a lot of other things, also.  Spiral is a very technical approach.  Agile, however, recognizes that technology is built by people.  The
Agile Manifesto
has four principles that are above and beyond the Boehm's simple risk management approach.