Game design is a complex process that often involves multiple people using different software (e.g. Photoshop, Unity and Maya, or GIMP, Unreal and Blender) to create different deliverables (e.g. sprites, 3D levels and 3D props). Consequently, it is necessary to break down the process into simple, manageable steps, e.g. a pipeline and/or workflow.
A 'pipeline' in a project management context is simply a sequence of tasks that are followed to create a product.
There is no single pipeline or workflow for the generation of assets or for their integration in a game, but there are some practical constraints based on project management and software that mean there are some broadly universal patterns in asset development and integration.
For example, before a 3D character asset can be imported, it needs to be animated, before it can be animated it needs to be rigged, before it is rigged it needs to be modelled, and before it is modelled it needs to be designed, and before it is designed you have to have a game concept. You can't employ someone to perform one of these roles if the person responsible for the prior role has not finished their task.
The following is a fairly standard overview of asset creation pipelines:
The term 'workflow' has a similar meaning to pipeline, but tends to be used when talking about the way you would use a particular program. For example, three different people might each have a different 'workflow' in Maya (a different process of what tools they are using, and how) as part of a shared pipeline to create a 3D character (one modelling, one rigging, one animating). Similarly, two artists may have two different workflows for how they create game UIs or character sprites in Photoshop, using different tools to create the look they want.
Throughout this unit you should be using more tools and finding a 'workflow' that works for you, and learning 'pipelines' that allow you to take a complex task, like making a game level, and breaking it into discrete steps or stages to make it manageable.