Forestry is quite a diverse mod, but I think the core concepts are:
1. Biofuel. The transformation of natural materials (saplings, wheat, reeds, etc.) into biomass, and further to biofuel is a concept added by forestry. It also adds both the biogas engine, and the biofuel generator. Biofuel itself is also able to burn in a combustion engine, for lower output and efficiency than the standard fuel (the tradeoff being that it's renewable).
2. Breeding. Bees (and, by extension, trees), are breedable. The outputs of bees range from bee-specfic products, and additional food items (with the vanilla forestry), to most diffcult-to-automate materials (oil, metals, dyes, etc.) with the addition of Extra Bees. Thaumic bees adds a number of additional effects and products, to do with thaumcraft, to the mix.
3. Automation. The farms within forestry allow the automation of previously manual tasks, at the expense of fertilizer and power. Many of the forestry-specific things can also be automated in this way.
There are a number of supplementary items that are also added (backpacks, electrical engines, etc.), details of which can be found on the forestry wiki (which I would link if it weren't late, and my bed was not calling my name).