Texture filtering is what gives the textures their blurry look, this can be good for trying to hide low resolution textures. However, some people dislike them or enjoy a more pixelated look. I will show you how to disable it below.

In order to disable texture filtering you must open the console by pressing the "~" key. (Make sure the developer console is enable in the advanced keyboard settings.) Then type "mat_filtertextures 0". The textures around you should become unfiltered and pixelated.

Before:
After:


You can also disable lightmap filtering by typing "mat_filterlightmaps 0". This causes the shadows to become blocky and has a cool retro look.

I hope you find this guide useful. Feel free to ask questions in the comments.