What are the types and pros/cons of electric and manual spice and nut grinders available? What qualities should be looked for while purchasing them?
No one device, manual or electric, is ideal for all of the tasks that you have enumerated.   Many cooks will have more than one tool, depending on the job at hand.

Some spices, particularly cinnamon, are very difficult to grind effectively at home without leaving fibrous bits that may give an unpleasant mouth feel.  Of course, when infusing flavor from a whole stick of cinnamon, this is not an issue.

Bottom Line

I would recommend having at least a Microplane type grater for ginger, garlic, and so on, and a rotary type electric grinder for most hard spices.  

For the specialty task of making peanut butter, a blender is probably your best tool.

Morter and Pestle



Also known as a molcajete, is probably the most versatile, but most labor intensive spice grinding tool.


Can grind, mash, or mix both dry or wet items, so it is suitable for garlic and ginger; still it is most effective on very hard, frangible spices which can be crushed, or soft items like garlic which can be mashed.  Fibrous spices like cumin can be very difficult to do.
Lots of work
Only suitable for small quantities


Microplane style grater



A grater with very fine, usually etched, openings is suitable for some tasks including:


Garlic, gingner, galangal and the like
Nutmeg


Nutmeg Grinder



A specialty item, just for nutmeg.  Most nutmeg grinders actually make very fine shavings, but it is essentially the same thing.  They also have storage for nutmegs.

Blender



Electric blenders are suitable for some grinding tasks:


Making peanut butter
Grinding soft spices or making wet spice mixtures, as with garlic, ginger, capiscum type peppers


Some high quality blenders may also do a decent job with softer seeds like cumin seeds.

Electric rotary grinder



These are often marketed as coffee grinders, but they are extremely effective as spice grinders&mdash;however, due to lingering flavors, you want to devote one to either coffee or to spices.

Suitable for dry spices, such as cumin, black pepper, allspice, mustard seeds, and so on.

Not very effective for the hardest spices, such as cinnamon.  

Note that clove oil (also found to some extent in nutmeg) can cloud plastic parts over time.

Pepper Grinder



As the name implies, ideal for black pepper, but also effective on other small spices that can fit through the mill such as mustard seeds.  Also provides a small quanitity of storage.  Most permit the grind size to be adjusted.

Specific Questions

Which kind of grinders allow water in them for making a paste of the spices?

Mortar and pestle, and blenders are suitable for pastes.  However, you can also grind your dry spices, and then add them to a wet mixture to create a paste.

Is there some maximum and minimum capacity for each of the type of grinders?

Most home style devices are suitable only for very small quantities.  The nutmeg and pepper grinders are good for teaspoon type quantities, and rotary grinders up to a few tablespoons.

Blnders have much higher capacities, but may not be effective except with very soft or frangible items.  Peanut butter is one application that they do well with in larger quantities.

What are the types of manual grinders which do not require too much amount of manual power/effort?

They all require significant effort.  For spices used in very small quantities, such as black pepper, that may not be noticeable.

Which kind of grinders allow us to have a fine powder of spices rather than bits?

None of the home methods will produce the very fine, even powder that you might expect from a commercial spice house.  They have extremely high quality, sometimes specialized equipment, and they also sieve or screen the resulting product to ensure it is uniform, and to exclude larger bits.

Some spices are more suitable for home grinding than others.  Ones that you should have little trouble with include black pepper, nutmeg, allspice, mustard seed.

Spices that are very difficult to grind at home include cinnamon.

Cumin, one of the most popular spices, is easy to grind, but difficult to get a perfectly smooth and uniform powder.  It tends to have little threads and fibrous bits.  In many applications this is not noticeable.