Wood decks are built with a structural system consisting of posts, beams, joists, decking, and railing often constructed in just that order. The lumber used if outdoors will need to be pressure treated to resist the following damages: UV Ray, moisture, rot, and insect caused. Most pressure treated lumber can receive paint (latex based) or stain (oil based) immediately, but often times it’s recommended to wait at least 6 months, if not a full season to let the lumber dry out and acclimate. After stain or paint, preserved wood will require routine maintenance (cleaning and restaining). Also, water repellant coatings can be applied to help lock out moisture and provide additional life and longevity.Composite Lumber (often shredded polyethylene) or Plastic Lumber (often polyvinyl chloride or ‘PVC’) can be used for a high-end look and low maintenance product.  
Wood Material Types:
Southern Tellow Pine – durable, strong, and environmentally friendly.
Hemlock – Uniform wood grain and natural decay resistance.
Cypress – Strong and natural resistance to decay, easy to work with.
Cedar – Resists moisture, resists rot and insect infestation. Reacts well untreated but a paint/stain is recommended.
Douglas-Fir – Strong and visually appealing, decay/termite/mold resistant
Redwood – decay, insect, warping, splitting resistant. Easy to work with
Exotic Species – More durable, beautiful, and expensive.
For a simple post on pier footer, the decking space above post carrying live and dead load (tributary) area is multiplied by the assumed load (typically 40psf live and 10psf dead). Example – 50psf x 20sf = 1000lbs. This area is found by dividing half the beam and joist spans on all sides of the post. Then based on the allowable soil capacity of the soil type (varying between 1500psf and 3,000psf) a calculation can be made to figure the area of footer required. Example – 1500psf / 1000lbs = 1.5SF footer surface area required for proper soil bearing. If you have a circular pier, you will have to calculate the area by way of Pi x Radius squared. Example – 0.75SF x 0.75SF x 3.14 = 1.76SF, this would be enough area to support the weight. Contractor’s will often find the largest diameter footer required based on loads and use that for simplicity and continuity. Wood and concrete touching does not bode well structurally over long periods of time, so localities may require a post base cast into the footer to receive the wood post.
If in frost zones, localities may require the bottom of the foundation be a minimum of 42″ below grade (this depth varies from location to location).
6×6 Posts are adequate for almost any standard deck design, but 4×4 posts are most commonly used and typically adequate for most simple decks.
The purpose of the beam is to transfer the joist loads to the posts and down to the footers into the soil. The allowable beam span will depend on the allowable deflection of the beam (determined by material type, thickness, and depth), total load (which is already largely addressed in the footer size design), and the span of the joists being attached to the beam. Rough sawn solid lumber can be used, but often 2x lumber is utilized by face-nailing pieces together creating a beam.
Below is a chart that can be used for a 50lbs/sf (40 live and 10 dead) deck design based on the use of southern yellow pine and the following relevant factors:
Joist Span Distance Between Posts
(2/ea) 2×6 (2/ea) 2×8 (2/ea) 2×10 (2/ea) 2×12
8′ 7′-9″ 10′-3″ 12′ 14′-3″
9′ 7′-3″ 9′-6″ 11′-6″ 13′-6″
10′ 7′ 9′-0″ 10′-9″ 12′-9″
11′ 6′-6″ 8′-6″ 10′-3″ 12′
12′ 5′-9″ 8′ 9′-9″ 11′-6″
13′ 5′-3″ 7′-3″ 9′-6″ 11′
14′ 4′-9″ 6′-9″ 9′ 10′-9″
15′ 4′-3″ 6′-3″ 8′-3″ 10′-3″
16′ 3’9″ 5′-9″ 7′-9″ 9′-6″
Beams can bear on the top of the posts with straps or ties or be lag bolted to the sides of the posts (must consider this in footer/post design).
The purpose of the joists are to transfer the live and dead loads from the decking boards on to the beams, posts, footers, and soil. 2x Lumber is commonly used. Joists are attached to the beams most commonly with joist hangers which are typically exterior rated and customized to fit joist depth and width.
The following is a chart that can be utilized for choosing joists based on southern yellow pine and other applicable factors:
Joist Spacing Maximum Joist Span
2×6 2×8 2×10 2×12
12″ 10′-9″ 14′-2″ 18′ 21′-9″
16″ 9′-9″ 12′-10″ 16′-1″ 18′-10″
19.2″ 9′-2″ 12′-1″ 14′-8″ 16′-1″
24″ 8′-6″ 11′ 13′-1″ 15′-5″
Corrosion-resistant fasteners should be used of: hot-dipped galvanized steel, stainless steel, or polymer coated steel among others. Two fasteners should be used across 2×4 decking, and 3 across 2×6 decking at the joist joint. Nails or screws can be used with nails being more efficient, but screws being stronger. If nails are used they should be driven into deck boards at a slight angle towards eachother.  
Decking boards should be separated for future expansion. With wet/heavy wood allow a max of 1/16″ spacing, if the wood is dry/light, allow max of 1/8″ spacing. If a decking board is bowed install it crown up and if a board is bent slightly it can be straightened during fastening. Generally with 24″ and 19.2″ joist spacing, 2″ decking boards are recommended. With 16″ joist spacing, 1-1/4″ (actual) decking thicknesses are recommended. With 12″ joist spacing, 1″ (actual) decking thicknesses are recommended. 
Worker installing wood floor for patio
Paints, Sealers, & Finishes
Paint – more logical with less expensive, less beautiful wood, particularly pine. A primer (often acrylic latex) which helps resists fading/peeling/cracking/scuffing. An unfortunate side effect of deck painting is flaking and peeling. Also painted decks will require recoating every 4-5 years.
Solid Stain – Opaque and rich, typically oil based with coverage similar to paint but penetrating characteristics of sealing. Wood tones and color options available. Generally more protective from weathering and sublight that paints over longer periods of time.
Semi-Transparent Stains – Contain a light pigmentation, penetrates grain of the wood to enhance the natural dynamics of the wood. Will fade over time and require a recoat every 1-3 years to maximize wood beauty.
semi solid deck stain
Tinted Waterproofer – protects against dampness, protects from UV rays, and slightly enhances color. Will require a recoat every 2-3 years.
Clear, Water Sealer – protects against dampness and protects wood while bringing maintaining natural wood color. Will require a recoat every 2-3 years.
Routinely sweeping and hosing off a deck will prolong life and minimize mold/mildew. Yearly cleaning is also good practice with a mild deck detergent and a stiff brush.