Sequential collation is the simplest way of ordering the cards from a sheet. It was originally used in early US printings of Magic sets, and is now used in all regions.
Each row of cards appears in order from left to right. The rows are ordered from top to bottom. The bottom right card will be followed again by the top left card.
Unlike striped collation, fillers may be used. Generally, fillers seem to appear at the end of the sheet (or at the end of the run for a sheet that has multiple runs).
In order to make the collation appear more random, each card will generally appear at least twice on a sheet that uses sequential collation (generally with somewhat different neighboring cards). A card will usually not appear on more than one different sheet or in multiple different runs (unless they can't appear together in the same pack) to prevent the card from appearing more than once in the same pack.
Because of the one-dimensional nature of this collation method with respect to the print sheet, it is can be hard to reconstruct the actual sheet layout. Therefore, generally only the runs will be given without respect to the actual sheet layout, although the sheet layout is closely related. (For example, in a width 10 sheet, cards that are 10 apart are in the same column, unless fillers throw things off.)
Sheets using sequential collation are generally front cut [1], and the ordering given here reflects that. A back cut sheet would have the reverse order. When giving sheet reconstructions, I am assuming front cut sheets. If the sheets were actually back cut, the sheet layout would be inverted.
[1] Information about corner cutting is from The Green Disenchant Project's "Corner Cutting" article.