XMPP is an open protocol for reliably- and instantly conveying text messages (one to one, or multi user akin to IRC), or initiating audio calls or video calls, or providing presence. XMPP was formerly named Jabber and continues to be reffered to as Jabber; XMPP accounts are Jabber IDs. Additionally XMPP can be secured painlessly with OTR, an open source encryption scheme requiring zero technical saavy. XMPP is capable of file transfer but one is better off using http or sFTP. XMPP has widespread industry adoption. While many individuals wield XMPP clients far more sheeple use XMPP without realizing it: F*book, TWITter, cisco, apple, microsoft, sony, google (google's xmpp is as almost as their imap), android, iphone. For more information see official xmpp wiki or official jabber FAQ.
XMPP clients are available on almost every internet capable platform. To find a mobile client search for "XMPP client" or "Jabber client"; most such apps are actually-free (if the app renders third party internet ads it is NOT-free but the adware flavor of malware).
XMPP (or SIP) replaces sms for texting
XMPP entirely replaces sms for texting; legacy-sms can be conveyed via XMPP or SIP SIMPLE and is currently available from several top tier VoIP service providers (like anveo.com, vitelity.com, and voip.ms). acrobits groundwire is a cross platform mobile SIP client capable of $0 texting (including legacy-sms), presence, audio calls; it is also SIP TCP capable meaning a lower battery impact than typical SIP UDP. Any webhost worth considering allows one to use the same address for: email, SIP, and XMPP.
XMPP vs SIP
A compelling alternative open standard is SIP: texting, audio calls, video calls, media delivery, file transfer, presence. Both SIP and XMPP have much feature overlap. XMPP has a well developed extension for managing privacy: XEP-0016. SIP content filtering relies upon the features of one's PBX.
Encryption is available for one to one texting, multiuser texting, audio calls. XMPP and SIP share OTR for one to one texting. Multiuser texting is only available in XMPP as is secured with mpOTR. Audio calls can be encrypted with zRTP with optional sRTP overlay.
sms for texting is not secure
By mandate of the US federal government EVERY sms text message is stored for all eternity. This violate's one's Fourth Amendment rights. No such unconstitutional provision is made for XMPP -- and would be rendered impossible for XMPP when using OTR.
system notifications via XMPP