This is what we're ignoring:
$ tree
.
|-- builds # compiled and seed stages
| `-- default
| |-- stage1-i686-2013.1.tar.bz2
| ...
| `-- stage3-i686-2013.1.tar.bz2.DIGESTS
|-- iso # compiled ISOs
| |-- swc-x86.iso
| |-- swc-x86.iso.CONTENTS
| `-- swc-x86.iso.DIGESTS
|-- kerncache # cached kernel compilation
| `-- default
| |-- livecd-stage1-i686-2013.1 # empty (no kernel in livecd-stage1)
| |-- livecd-stage2-i686-2013.1 # lots of kernel compilation stuff
| ...
|-- packages # binary packages from per-stage emerges
| `-- default
| |-- livecd-stage1-i686-2013.1 # emerged during livecd-stage1
| ...
| `-- stage3-i686-2013.1 # emerged during stage3
...
|-- snapshot_cache # unpacked portage snapshots
| `--
20130208
|-- snapshots # packed portage snapshots
| |-- portage-
20130208.tar.bz2
| |-- portage-
20130208.tar.bz2.CONTENTS
| `-- portage-
20130208.tar.bz2.DIGESTS
...
`-- tmp # unpacked stage chroots
|-- default
| |-- livecd-stage1-i686-2013.1
| |-- livecd-stage2-i686-2013.1
| |-- stage1-i686-2013.1
| |-- stage2-i686-2013.1
| `-- stage3-i686-2013.1
`-- portage
`-- portage # unpacked portage snapshot
--- /dev/null
+builds
+iso
+kerncache
+packages
+snapshot_cache
+snapshots
+tmp