Today, it's Back to Basics time!
There are many ways to animate your avatar. The easiest is to use the built-in animations...the ones that make you walk, and sit, and stand. In fact, it's automatic! But the built-in animations are pretty dorky, and most people replace them with better animations using a gadget called an animation overrider, or AO.
Your start-up inventory also includes some dance animations. You can find them in your Library/Animations folder. These are also pretty dorky, so when you go out dancing, use the dances provided in the club's dance machine.
To play these stock dance animations, or any animation you receive in "raw" form, you can double click the animation item in your inventory to open it on your screen, then click "Play in World". You will continue to be animated until you click Stop. (Play on Screen also works, but only you will see the animation.)
As you wander about Second Life, you will undoubtedly encounter some spheres here and there, about the size of a soccer ball. They are usually colored pink (for girls) and blue (for boys). Sometimes they are white (unisex). These are "poseballs". If you right click one and Sit on it, your avatar will be animated with whatever animation the poseball contains. Very often, these poseballs occur in pairs. When you and a friend sit on them, they will animate you both in a couples animation...a cuddle or a kiss perhaps. Or a dance...or even an XXX-rated sexual animation. NOTE: You should turn off your AO, and you may get an animation request notice from the poseball which you must accept before it will animate you.
Some poseballs are temporary. They appear above a bed or other piece of furniture when you click the furniture and choose an animation from its menu. When you get off the bed, and click the Stop choice in the menu, the poseballs disappear again.
You must be in a place that allows you to rez items (create them in world) to use your own poseballs, whether they are simply items in your inventory, or rezzed from a device you are wearing, like a "romance HUD". However, you can use poseballs in existing furniture, provided that the furniture has been set to allow you as a user. (Some furniture is limited by its owner to owner-only operation, or group-only operation.)
To stop using a poseball, just right click yourself and select Stand, or click the Stand button near the bottom of your screen. Or, if the poseballs are rezzed by a dance machine or a piece of furniture, you can also choose Stop from the controlling item's menu.
Poseballs may be hidden, so as to avoid the unrealistic situation of colored spheres floating all over the place. Type "/1 hide" in local chat to hide them, or "/1 show" to make them appear (don't type the quotation marks.) Some poseballs "listen" on other chat channels, but channel 1 is the most common.
Poseballs are especially useful for couples animations, because you can edit the relative positions of the participants by editing the position of the poseballs. So you can tweak them to allow your lips to meet in that kiss animation. Some furniture menus have special menu selections to make this easier, but you can also select the poseball for editing, then sit on it. Even though the poseball becomes invisible, the edit menu will remain active, and you can adjust the ball's position with the positioning arrows.
Poseballs most often contain animations...that is, your avatar will move through a realistic sequence of actions. However, for photographic purposes, you don't want your avatar to move, you want her to POSE. There are special animations that are just that...static poses. Although they have the same symbol as any other animation (a little running figure), they do not move your avatar, but keep her in one position. You may find poseballs with static poses like this in photography studios in Second Life, and there are some stores that specialize in selling poses, primarily to the model community.
You can make your own poseballs. Here is a link to a poseball script. Follow the directions to download it, and upload it to SL. Then drop it, and an animation, into a prim. The prim may be any size or shape, but you may as well follow tradition and make it a sphere. Edit the script so it will recognize your animation by name. Then...hey, sit on it!
A NOTE TO TINIES AND NONHUMAN AVATARS: Poseballs, and any animation or pose intended for a human avatar can mess you up, stretching your body to try to get it back to a human shape. This is not serious, you will go back to normal when you stand up or stop the animation...but it can be frustrating.
ETIQUETTE NOTE: Sex bed stores have samples out on the floor for you to try before you buy. If you do try these out, DO NOT take off your clothes or put on your prim genitals! Most stores will ban you if they catch you actually using their floor samples for cyber-sex. Geeze...get a ROOM! And guys...the pickup line, "Would you please help me try out this bed's poseballs?" is VERY lame. You Have Been Warned!