Encumbrance is the load you are carrying. Maximum encumbrance is a derived attribute that determines how much weight you can carry. The total weight of all the items in your inventory must always be less than your maximum encumbrance. Otherwise, you will become over-encumbered and will not be able to move. Grabbed items do not count against your encumbrance. Your maximum encumbrance (without any magic effects applied) is five times your Strength.
Total weight carried has no effect on your Speed, but total weight equipped (armor and weapon) can have a significant impact on it (and especially so if your equipped weapon is drawn). Your maximum encumbrance can be temporarily or constantly increased using a Feather or Fortify Strength spell or enchantment. Contrary to how it is reported in game, Feather actually decreases your current encumbrance rather than increasing your maximum encumbrance. This results in a limit on the total magnitude of active Feather effects equal to your encumbrance at the time the last Feather effect was applied. On the other hand, it also means that Feather effects allow you to run faster by decreasing the weight of your equipped armor and weapon, whereas Fortify Strength will not increase your speed.
Encumbrance can be a very annoying limitation. This is especially true when you have a lot of loot and wish to sell it in a distant city. Fortunately, there are a few ways to get around encumbrance limits with a little preparation, so that you can massively overencumber yourself and still be able to move.
 Carrying Things In Front of You
As mentioned above, grabbed items do not count against your encumbrance. If you find yourself overencumbered and have a heavy item in your inventory (or especially if you have a big stack of identical medium-weight items), you can drop and grab that item (or stack) and you will be able to move again (if what you dropped gets you back under the weight limit, of course). Depending on what button you use to do the grabbing action, you may need to continually hold that button for as long as necessary. One important thing to know is that you can still go through doors when overencumbered. So you can walk up to a door with a grabbed item, take it back into your inventory, go through the door, then drop and grab the item again to regain the ability to move.
However, there is a very useful way to cheat a little using this whole "grabbing" technique. There are four types of portable containers in the game (also known as "low-weight dead bodies"). In order of increasing carrying difficulty (weight) they are:
1) Slaughterfish, 2) Rats, 3) Skeletons, 4) Wolves.
You can load up one of these dead things with thousands and thousands of pounds of items, and then pick the body up and carry it for miles, by grabbing and lifting. The technique is not exactly simple:
- to lift a body you need to guess where the spine is, and grab a little below the head (rats can be lifted at the base of the tail),
- to carry a body uphill without dropping it, you usually must walk very slowly (the heavier the body, the slower you must move),
- you have to hold the "grab" button continuously (sometimes for hours),
- you cannot ever carry a dead body through a door or city gate.
The advantage to this carrying technique is that when you get attacked you can drop the dead body and fight. Then loot all the attackers, toss all the loot into your dead body container, pick it up, and start traveling again. Also, carrying smelly dead things around is an indication of class and distinction.
The main disadvantage to this method is speed. When carrying a wolf uphill, you can barely move at a crawl or you will drop it. An additional small disadvantage is that dead slaughterfish, rats and wolves are unsafe containers if you must put them down and leave the area for more than 72 hours. If you put down a skeleton and leave the area, then return -- there is a small chance the skeleton may temporarily cease to exist (you have to leave and come back again). There is also a small chance a skeleton may turn permanently invisible.
Horses have no encumbrance limits. The problem is getting on and off the horse.
If you are over-encumbered, you cannot mount a horse. You can obviously use a combination of short-term Feather and strength fortification to get yourself into the saddle. But there is one other possible trick mentioned below.
If you become over-encumbered while you are on a horse, that is not a problem until after you dismount and suddenly become immobile.
To take advantage of this, you need to load up your inventory while you are already mounted on the horse. To do this, you need items or containers that are up high, where you can reach them while you are already mounted. One way is to balance stacks of items on top of fence posts or rocks -- this can be annoyingly difficult and takes hours to do. An easier way is to lift and place a portable container (e.g. a dead body) on top of a rock (skeleton arms work nicely for this) or on top of a wall or well. Also: in the vineyards of Skingrad, at some inns with stables, and in Waterside and Weye, there are normal containers that are high enough to be accessed from horseback. Pick a container, load umpteen tons of items into it, ride up to it, take it all into your inventory, and gallop off.
The dead (as opposed to "undead") skeletons that you find outdoors can be very valuable in this way, since they are portable, they (mostly) stay where you put them forever, you can still sometimes move the arms and skulls short distances, and skeletons are safe containers.
Once you have completed most of the Dark Brotherhood questline, you get the special unkillable horse Shadowmere. That is, the horse can temporarily die but then she will get better. While Shadowmere is dead she becomes a container. So you can kill her yourself, toss all your loot into her dead body, wait until she recovers, travel to your destination, kill her again, and take your stuff out. Or you may consider this glitch to be aesthetically uncool.