Jeff Cohen has a nice comparison of Kanban vs Scrum. In a nutshell, Kanban emphasizes pull- instead of push-based queues which, among other benefits, is better at handling non-brick-laying development (i.e. new tools, new languages, new problems) where time estimates are hard to judge. From Jeff Cohen:

Goal/Theme: Maximize flow through your development department by introducing the concept of limits.
Can work alongside scrum in terms of requirements mangement; but replaces the predictive, timeboxed, pushed-work sprint system with a series of small work queues from which all team members “pull” tasks whenever they’re ready for new work. Velocity is still a measurement of how many features/bugs/tickets that can flow from the starting queue to the final queue in a given week or month.
Kanban does not prescribe any meetings; the ability of the team to figure out their own communication style is assumed. Many teams adopt the estimation and prioritization concepts from scrum to manage the requirements and decide what goes into the kanban queues. Search google for “scrumban” to get a variety of opinions on this.
+ drives progress within a team or across multiple teams equally well
+ the notion of limited work queues tend to improve focus and remove excessive context-switching
+ no sprint predictions: team members pull new work whenever they’re ready for more
+ deployment/release management not a special step, but simply another kanban queue
+ features can be released can occur in step with, or out-of-band of, ongoing development
+ engineers can work on short tasks, or long-running tasks, without the limited timebox of a scrum sprint
– no prescription for how to manage requirements
– no opinion on management technique; many kanban teams choose a simplified version of scrum
– some people think it’s “scrum without sprints,” but that underestimates the subtle but important difference between pull* – vs. push-based work scheduling

The article also compares the other methodologies such as Waterfall, XP, and everyone’s favorite: Chaos.