TOC stands for Table of Contents. It is a required element in ePub files and very important in most eBook formats.
A TOC is available in nearly every published book and thus becomes a key feature of eBooks as well. Some books also have secondary TOC's that supplement the main one. This include, but are not limited to Lists of Figures, LOF (sometimes called Table of Figures), List of Tables, LOT, and segmented TOC's that sometime appear within chapters. In eBooks these are typically hot links so that tapping or clicking the entry will go directly to that place in the eBook. A TOC will sometimes have an hierarchy with section entries inside a chapter.
 Inline TOC
An inline toc is a toc built as a page within the eBook itself just like a paper book. This is the way you would do it for an eBook in MOBI format and optionally in other formats. The TOC in an eBook differs from the paper version in that it can have links embedded in the TOC titles to permit the eBook reader to jump to the desired chapter or section. For this reason there are generally not page numbers in an eBook TOC.
Inline TOCs can automatically be created by many word processors such as Word using style semantics. The links are created as well.
The advantages of an Inline TOC are that there can be multiple ones and they most resemble the paper book. Inline TOC can also have additional descriptive text embedded inside the TOC.
The disadvantage is that, without an external reference, it can take several clicks to find them. Some users set a bookmark on the TOC for easy reference.
 Separate TOC
EPub requires a separate TOC. For ePub 2 it in an NCX file that is built as part of the ePub collection of files. For ePub 3 an optional NCX file can be added for compatibility purposes but the standard is for an XHTML file with a TOC semantics entry to identify it. The same TOC can be referenced externally and inline if desired.
PDF calls it bookmarks and lists it in a special pane on the left side.
 MOBI TOC
The MOBI format TOC is in the HTML file but should be identified with an id="toc" entry at the top of the contents area. A guide entry is also needed. Mobi is similar to other OEB eBooks like LIT and IMP.
 Kindle TOC
 PDF TOC
A PDF does not require a TOC but can support an inline and/or a separate one. The separate one is called bookmarks and will appear in the left column (usually collapsible) in the PDF reader and can be used to jump to the desired chapter or section. Some PDF generation tools do not support either form of linked TOC or other links for that matter. This is primarily the case for PDF files generated as a "Print to file" command.