A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. Typical galaxies range from dwarfs with as few as ten million stars, up to giants with one trillion stars, all orbiting the galaxy's center of mass. Galaxies can also contain many multiple star systems, star clusters, and various interstellar clouds. The Sun is one of the stars in the Milky Way galaxy; the Solar System includes the Earth and all the other objects that orbit the Sun. There are probably more than 100 billion galaxies in the observable universe. Source: Wikipedia
Our own galaxy, the Milky Way, is 100,000 light years across. The nearest large spiral galaxy like the Milky Way is the Andromeda galaxy, about 2.7 million light years away. As you can see, the Universe is a very large place. Check out this YouTube video to get some scale on the size of Earth. Remember these are only stars, and that each galaxy has many many millions of them!
A nebula is an interstellar cloud of dust, hydrogen gas, helium gas and plasma. In these regions the formations of gas, dust and other materials 'clump' together to form larger masses, which attract further matter, and eventually will become big enough to form stars. The remaining materials are then believed to form planets, and other planetary system objects. Many nebulae form from the gravitational collapse of gas. As the material collapses under its own weight, massive stars may form in the center, and their ultraviolet radiation ionises the surrounding gas, which creates plasma, making it visible at optical wavelengths. These are sites where star formation occurs. The formed stars are sometimes known as a young, loose cluster.
Some nebulae are formed as the result of supernova explosions, the death throes of massive, short-lived stars. The materials thrown off from the supernova explosion are ionized by the energy and the compact object that it can produce. Other nebulae may form as planetary nebulae. This is the final stage of a low-mass star's life, like Earth's Sun. Stars with a mass up to 8-10 solar masses evolve into red giants and slowly lose their outer layers during pulsations in their atmospheres. When a star has lost a sufficient amount of material, its temperature increases and the ultraviolet radiation it emits is capable of ionizing the surrounding nebula that it has thrown off. The nebula is 97% Hydrogen and 3% Helium with trace materials. The main goal in this stage is to achieve equilibrium. Source: Wikipedia
Narrowband filters are designed to capture specific wavelengths of light. There is a large class of celestial objects known as emission nebulae, and their name arises from the fact that they are actually emitting their own light (as opposed to reflection nebulae, which shine by reflected starlight). Planetary nebulae are normally considered a separate class of objects than emission nebulae, since they represent a very different phenomenon (star death instead of star birth), but for CCD imaging purposes, they can also be considered emission nebulae as they are emitting their own light. Supernova remnants also fall into this category, so objects like the Ring Nebula, Dumbell Nebula, Veil Nebula, and Crab Nebula are all potential targets for narrowband imaging as well.
Narrowband filters do not attempt to replicate the spectral sensitivity of the human eye. Therefore, color images created from these filters are called false color images. Typically, three filters are used and each is assigned to one channel of an RGB image. One filter becomes the red part of an image, one becomes the green part, and the third is the blue part. Once combined, each color represents a particular wavelength of light and hence a particular element in the gas cloud. In addition to being a pretty picture, a narrowband image is also scientifically interesting (which is why these types of filters are used on the Hubble Space Telescope and other professional instruments). Source: Starizona
Star clusters or star clouds are groups of stars. Two types of star clusters can be distinguished: globular clusters are tight groups of hundreds of thousands of very old stars which are gravitationally bound, while open clusters, a more loosely clustered group of stars, generally contain less than a few hundred members, and are often very young. Globular clusters are roughly spherical groupings of from 10,000 to several million stars, packed into regions of from 10 to 30 light years across. They commonly consist of very old stars, just a few hundred million years younger than the universe itself. Source: Wikipedia