"Ubuntu is officially compatible with three very common processor architectures - x86 (aka i386), AMD64 (aka x86_64) and PowerPC.
This covers the vast majority of desktop PCs and small-to-medium servers. If you're blissfully ignorant of the technical details of your computer, you most likely have this architecture.
Technically, this includes nearly all x86-based processors, including AMD and VIA (former Cyrix) processors, and newer processors like Athlon XP and Intel P4 Xeon. (286 and earlier processors are *not* supported by Ubuntu, or even Linux in general).
This architecture also covers nearly all bus systems that were ever used with these processors: ISA, EISA, PCI, MCA (Microchannel), and VESA Local Bus (VLB).
Note that Laptops may contain specialized or proprietary hardware; to check whether your laptop works well with Ubuntu, take a look at the Linux Laptop Pages.
You can use either x86 or x86_64 with a 64 bit processor, see advantages/disadvantages to running x86_64.
This covers AMD processors with the "amd64" extension and Intel processors with the "em64t" extension.
Please note that the ia64 architecture is for Intel Itanium Processors Only. (Intel's "ia64" architecture is different. Ubuntu doesn't officially support ia64 yet, but work is well underway, and many Ubuntu/ia64 packages are available as of 2004-01-16).
A list of supported mainboards can be found at https://alioth.debian.org/docman/view.php/30192/27/mainboards.html.
64 bit ARM SoCs (System on Chip) from multiple SoC vendors have been enabled and certified on Ubuntu 14.04 and newer releases. Please refer to https://www.ubuntu.com/download/server/arm for download and installation instructions. A list of Ubuntu certified ARM SoCs and Servers can be found on Ubuntu the SoC Certification (see https://certification.ubuntu.com/soc/) and Ubuntu Server Certification pages (see https://certification.ubuntu.com/server/).
POWER (OpenPOWER) Servers
As of Ubuntu 14.04 LTS, IBM's POWER8 (ppc64el) architecture and its OpenPOWER variants are fully supported. Please refer to http://wiki.ubuntu.com/ppc64el for details.
S390X (IBM Mainframe Systems)
As of Ubuntu 16.04 LTS, certain systems that utilize IBM's S390X architecture, also referred to as "System z" and "LinuxONE" are fully supported. Currently, these are: IBM z13, IBM z13s, IBM zEC12, IBM zBC12, LinuxONE Emperor and LinuxONE Rockhopper. Please refer to https://wiki.ubuntu.com/S390X for details.
Please note, that PowerPC Long Term Support ended with Ubuntu 6.06. Updates were still available from Ubuntu Ports until Ubuntu 16.04.
This covers the majority of machines with a PowerPC CPU.
Most prominent are the Power Macintoshes produced by Apple. Note that older Macs with a Motorola 680x0 processor are *not* covered here. (They have the "m68k" architecture, which isn't supported by Ubuntu but may be supported by other Linux distributions.)
The PowerPC family is quite varied though, you can find everything from Power Macintosh clones to IBM small-to-medium servers here.
This architecture also covers most bus systems that are in use with the PowerPC processor. The most notable exception is the NuBus, which was used in some Apple Macintosh models (before Apple switched over to PCI). (A Linux kernel and limited support for the NuBus can be found at http://nubus-pmac.sourceforge.net/).
Here's a list of supported models:
Manufacturer: Apple (Linux Subarchitecture: powermac-NewWorld)
- iMac Bondi Blue, 5 Flavors, Slot Loading
- iMac Summer 2000, Early 2001
- iBook, iBook SE, iBook Dual USB
Power Macintosh Blue and White (B&W) G3
- Power Macintosh G4 PCI, AGP, Cube
- Power Macintosh G4 Gigabit Ethernet
- Power Macintosh G4 Digital Audio, Quicksilver
PowerBook G3 FireWire Pismo (2000)
PowerBook G3 Lombard (1999)
PowerBook G4 Titanium
Manufacturer: Apple (Linux Subarchitecture: powermac-OldWorld)
- Performa 4400, 54xx, 5500
- Performa 6360, 6400, 6500
- Power Macintosh 4400, 5400
- Power Macintosh 7200, 7300, 7500, 7600
- Power Macintosh 8200, 8500, 8600
- Power Macintosh 9500, 9600
- Power Macintosh (Beige) G3 Minitower
- Power Macintosh (Beige) Desktop, All-in-One
PowerBook 2400, 3400, 3500
PowerBook G3 Wallstreet (1998)
- Twentieth Anniversary Macintosh
- Workgroup Server 7250, 7350, 8550, 9650, G3
Manufacturer: Power Computing (Linux Subarchitecture: powermac-OldWorld)
PowerBase, PowerTower / Pro, PowerWave
PowerCenter / Pro, PowerCurve
Manufacturer: UMAX (Linux Subarchitecture: powermac-OldWorld)
- C500, C600, J700, S900
Manufacturer: APS (Linux Subarchitecture: powermac-OldWorld)
- APS Tech M*Power 604e/2000
Manufacturer: Motorola (Linux Subarchitecture: powermac-OldWorld)
- Starmax 3000, 4000, 5000, 5500
- Manufacturer: Motorola (Linux Subarchitecture: prep)
Firepower, PowerStack Series E, PowerStack II
- MPC 7xx, 8xx
- MTX, MTX+
- Manufacturer: IBM (Linux Subarchitecture: prep)
- RS/6000 40P, 43P
- RS/6000 Power 830/850/860 (6070, 6050)
- RS/6000 6030, 7025, 7043
- RS/6000 p640
- RS/6000 B50, 43P-150, 44P
- Manufacturer: Amiga Power-UP Systems (Linux Subarchitecture: apus)
- A1200, A3000, A4000
This list is not exhaustive. New models are constantly being brought out, and some older models may be absent because they haven't been tested.
All of the above architectures may come in a multiprocessor flavor (often called "symmetric multiprocessing" or SMP). This is not a problem for Ubuntu: it can handle multiple processors just fine.
The installation process may choose to use just one of the CPUs, but that's no cause for alarm: you'll be given the opportunity to adapt Ubuntu to your hardware later, from this page: CompileTheKernel.
Sparc processors were supported from 6.06, 7.04, 7.10 on the server edition only. As of 8.04, Sparc is no longer commercially supported. It was maintained as an Ubuntu Port through 10.04.
Ubuntu doesn't currently support any other architectures. This doesn't mean that Ubuntu won't run - in fact it is entirely possible that it *will* install without a problem. After all, Ubuntu is based on the Debian distribution, which has support for eight additional architectures.
However, if you run into problems, the Ubuntu people may or may not be helpful in getting you going - no promises here.