You can use tab to switch between enemies

Is it possible to switch to your marked ally/enemy with a button?

Switching between allies would be awesome too..

Oops, my information was wrong. Changed my key bindings.

On the keyboard I know you can use the F1 - F4 buttons to switch between party members. Haven't tried it for the raids, though, so I don't know if the F5 - F8 buttons work for it, too.

There are several methods of targeting enemies/allies. One way is using the Tab-key to cycle through the targets and the Shift+Tab-key to cycle through them in a reverse order. But, there are also more direct keys to select certain targets, like:

• Cycle Through Enemies (Nearest to Farthest) -> Tab
• Cycle Through Enemies (Farthest to Nearest) -> Shift+Tab

• Target Self -> F1
• Target Members in Party List -> F2 - F8
• Target Pet/Familiar -> F9
• Target the Target of Your Current Target -> T
• Target Focus Target -> F10
• Target Nearest Enemy -> F11
• Target Nearest NPC or Object -> F12

• Cycle Up Through Enmity List -> Ctrl+NUM8
• Cycle Down Through Enmity List -> Ctrl+NUM2
• Cycle Through Alliance List (Forward) -> Ctrl+NUM6
• Cycle Through Alliance List (Backward) -> Ctrl+NUM4

Next to that you also have the following 3 keys to assist you with targeting:

• Face Target -> F
• Set/Clear Focus Target -> Shift+F
• Target Filter -> X *

* When pressed, the target filter will render players, NPCs, enemies, or objects untargetable according to your settings. Although disabled by default, it can be enabled by checking the box for "Enable target cycling" on the Filters tab under Control Settings in the Character Configuration menu.

Personally i limit my targeting to the Tab and the Shift+Tab keys and use my mouse to select specific party members within the party list.

I believe there is also a macro (in the user macros menu section of the game, there's like 6 pre-created ones) that is set up for targeting marked targets.

I use Shift + a mouse key on the side of my mouse to "Target target's target".
I use the same key without Shift to turn cleric stance on / off.

This is extremely useful as a WHM, because it allows me to immediately begin attacking whatever the tank is attacking. Also, as long as the tank has hate, pressing the key again will switch me from the mob back to the tank. Pressing the key without shift switches cleric stance, allowing me to heal / DPS. The mouse key is linked to a hotkey in the hotbar.

So I start with Shift + mouse key = target tank's target.
Mouse key = CS on
DPS DPS DPS
Shift + mouse key = target tank (because the target of the mob is the tank)
Mouse key = CS off
Heal
Rinse and repeat.

When I need to target other mobs or party members I use tab or F3, F4 respectively.