Domain Driven Design vs Model Driven Architecture
The root of both Domain-Driven Design (DDD) and Model Driven Architecture (MDA) is Model-Driven Engineering(MDE), also known as Model-Driven Software Development (MDSD) if limited to the software development domain. See Wikipedia: http://en.wikipedia.org/wiki/Model-driven_development
All approaches falling under the MDE umbrella have one thing in common: a model. How this model is materialized depends on the specific MDE flavor.
MDA is regarded as overly complex. DDD is considered by some as too abstract. My personal favorite MDE implementations are DSM and ABSE (not listed on the Wikipedia article).
DDD is about approaching a software solution from a business perspective with the intent of keeping the design as much close to the real world as possible. This is more of an art than engineering.
MDA solves different set of problems. More details here: http://xml.coverpages.org/OMG-MDAFAQfinal1.pdf
Don't disagree with most of the above although it's perhaps worth expanding a little.
The single most important concept in DDD is to focus on the problem domain. To put technology obsession to the side and concentrate primarily on modelling the problem you're trying to solve. So put ajax, ORMs, databases, frameworks etc. into the background and instead make sure you have a complete, accurate model of the problem first and foremost. (Of course you still need the architectural components - but they're explicitly subservient to the model). DDD calls this "Ubiquitous Language" - a model expressed in terms domain experts and developers alike use and understand. A model where the names of classes, methods etc. are taken from the problem domain.
DDD doesn't mandate /how/ you capture that model, although the book implies using an OO language to do so.
MDA shares that same notion of modelling the problem domain first and foremost (the PIM, Platform-Independent Model). As opposed to DDD, it recommends creating that model with UML. But the intent is the same: understand the problem domain without tainting it with (software) architectural concerns.
MDA's PSM (Platform-Specific Model) is somewhat analogous to applying the architectural patterns in DDD (e.g. aggregate, repository, etc.). Again - while different in specifics - both aim to solve the problem of converting a 'pure' problem domain model into a full software system.
So summing up, I'd say they are similar in two ways:
- The centrality of the Model (as @Rui says) - specifically the /Domain/ model.
- Applying architectural patterns to the model in order to realise the target system.
hth.